American Political Science Review

Research Article

Should I Stay or Should I Go? Explaining Variation in Western Jihadists' Choice between Domestic and Foreign Fighting

THOMAS HEGGHAMMER 

Norwegian Defence Research Establishment (FFI)

Abstract

This article studies variation in conflict theater choice by Western jihadists in an effort to understand their motivations. Some militants attack at home, whereas others join insurgencies abroad, but few scholars have asked why they make these different choices. Using open-source data, I estimate recruit supply for each theater, foreign fighter return rates, and returnee impact on domestic terrorist activity. The tentative data indicate that jihadists prefer foreign fighting, but a minority attacks at home after being radicalized, most often through foreign fighting or contact with a veteran. Most foreign fighters do not return for domestic operations, but those who do return are more effective operatives than nonveterans. The findings have implications for our understanding of the motivations of jihadists, for assessments of the terrorist threat posed by foreign fighters, and for counterterrorism policy.

Correspondence

Thomas Hegghammer is Senior Research Fellow, Norwegian Defence Research Establishment (FFI), P.O. Box 25, 2027 Kjeller, Norway (hegghammer@gmail.com).

Footnotes

  I thank Scott Gates, the APSR editors, and the anonymous reviewers for very useful comments on early drafts of the article. I also received helpful comments and advice from Kristin Bakke, Jeff Colgan, Lynn Eden, Karen Greenberg, David Laitin, Brynjar Lia, Yassin Musharbash, Vipin Narang, Petter Nesser, and Truls Tønnessen. I presented early versions of the article at University College London, the University of Aberdeen, and Stanford University; I thank all the participants in these seminars for helping improve the article. I am grateful to Stanford University's Center for International Security and Cooperation for the Zuckerman Fellowship that allowed me to complete the article.

List of Figures and Tables

TABLE 1.

TABLE 1. Estimated supply of Islamist domestic fighters in the West, 1990–2010, based on JPW dataset

TABLE 2.

TABLE 2. Estimates of the supply of Islamist foreign fighters from the West, 1990–2010, based on observation set

TABLE 3.

TABLE 3. Estimated supply rates and ratios of Islamist domestic and foreign fighters in Europe and the United States before and after September 11, 2001

TABLE 4.

TABLE 4. Declared initial purpose of first trip abroad by foreign-turned-domestic fighters

TABLE 5.

TABLE 5. Effect of veteran participation on domestic plot effectiveness

Why do some Western jihadists attack at home while others join insurgencies in places like Afghanistan and Somalia?1 In this article, I explore this variation empirically to shed light on the motivations of radical Islamists. Many assume that jihadists all want to attack the West, and that those who leave do so for training. I argue the opposite, namely, that most Western jihadists prefer foreign fighting, but a minority attacks at home after being radicalized, most often through foreign fighting or contact with a veteran. My tentative data indicate that militants usually do not leave intending to return for a domestic attack, but a small minority acquire that motivation along the way and become more effective operatives on their return. This has implications for counterterrorism, especially for the handling of foreign fighters.

This article addresses both a social science puzzle and a policy problem. The puzzle is the unexplained variation in choice of attack location. If jihadists have similar aims, why the different travel patterns? The policy problem is that of assessing the domestic terrorist threat posed by those who leave. Put bluntly, should foreign fighters be treated as lethal terrorists-in-the-making or as harmless freedom fighters? In the past, countermeasures have vacillated between lenience (pre-9/11) and harshness (post-9/11), depending on prevailing assumptions about what foreign fighters “really” want. Underestimating the threat is dangerous and overestimation expensive, so policy makers need assessments grounded in facts.

This article seeks to provide an empirical base from which to begin answering such questions. Using a variety of open-source data, I generate tentative estimates of the number of domestic and foreign fighters, the proportion of foreign fighters who return, and their impact on attack effectiveness. I also offer an explanation for the overall relative distribution.

The analysis focuses strictly on variation in theater choice by radicalized Islamists. Excluded is the question of pre-choice radicalization (why they radicalized) or post-choice tactics (how they fight). More important, I do not purport to explain individual differential choice because I lack biographical data on the foreign fighters. I only seek to account for the overall relative distribution, a task similar to that of the analyst who tries to understand an election outcome without having demographic data on all the voters. However, at the end I tentatively probe the question of who stays and who goes. Empirically, the inquiry focuses on jihadists in North America, Western Europe, and Australia between 1990 and 2010.

I employ ad hoc terminology in which a “domestic fighter” is a person who perpetrates or tries to perpetrate violence in the West, whereas a “foreign fighter” is someone who leaves or tries to leave the West to fight somewhere else. “Domestic” here means “anywhere in the West,” so a French jihadist attacking in Germany counts as a domestic fighter. “Foreign fighting” includes any military activity (training or fighting), using any tactic (terrorist or guerrilla tactics), against any enemy (Western or non-Western)—so long as it occurs outside the West. Co-ethnic war volunteers (e.g., American-Iraqis going to Iraq) are counted as foreign fighters.2

From here, the analysis proceeds in seven parts. The first reviews the literature. The next two explain how I generated estimates of domestic and foreign fighters. The fourth section describes the relative distribution, whereas the fifth offers explanations for it. The sixth part examines the effects of foreign fighting on domestic militancy, and the final part probes differential choice.

DOMESTIC AND FOREIGN FIGHTERS CONFLATED

The now substantial academic literature on Islamist militancy in the West has focused on the causes of individual radicalization (Dalgaard-Nielsen 2010). “Radicalization” has no established definition, but is usually understood as the process by which an individual acquires the motivation to use violence. The outcome of this process is usually conceptualized as a single phenomenon, variously termed “terrorism,” “militancy,” “violence,” “extremism,” “jihadism,” and the like. Few have distinguished analytically between different types of violent activism, and even fewer have asked why jihadists fight in different places.

The conflation of domestic and foreign fighting has distorted measurements of terrorist activity in the West. Several studies of “homegrown terrorism” in the United States include cases of foreign travel in their incident count, thereby overestimating the extent of domestic plotting. For example, 8 of the 22 “terrorist plots” in the study by Bjelopera and Randol (2010) are cases of people leaving the United States to join insurgent groups abroad. Similarly, 15 of the 47 “cases of domestic radicalization and recruitment to jihadist terrorism” in the work by Jenkins (2010) involve people seeking to fight abroad. Of the five cases of “homegrown extremism” in Nelson and Bodurian's study (2010), two involve people trying to go abroad. Other studiesalso conflate foreign and domestic fighting (Kurzman 2011; New America Foundation 2011; Schanzer, Kurzman, and Moosa 2010).

The difference between domestic and foreign fighting has not gone unnoticed, however. Taarnby (2005: 7), Silber and Bhatt (2007: 9), and Bjelopera and Randol (2010: 2) all observe that foreign fighters do not always end up attacking in the West. Kurzman, Schanzer, and Moosa (2011: 466) point out that several terrorism cases in the United States involved plots against targets abroad, and Dahl (2011) explicitly codes for location in his data on anti-U.S. terrorist plots. Brooks (2011:10) notes that “the category [of homegrown terrorism] encompasses individuals involved in a range of terrorist activities, including training with or joining foreign insurgencies,” and Hemmingsen (2010) distinguishes between “classical jihadists” (foreign fighters) and “global jihadists” (domestic fighters) in the West, noting that the “conceptualization [of jihadism in the West as one phenomenon] is not accurate” (2010: 169).

Moreover, in the literature on jihadism outside the West, the term “foreign fighter” has long been used to denote outside participants in Islamist insurgencies (Krueger 2007; Tumelty 2006). Some conceptual work has also been done to distinguish foreign fighter activism from terrorism and other forms of militancy (Hegghammer 2011; Malet 2009; 2010; Mendelsohn 2011; Noonan 2010). Empirical work on foreign fighters has focused either on recruit demand—why some conflicts and not others attract foreign fighters (Malet 2009)—or on the effect that foreign fighters have on the conflicts they join (Bakke 2011). The supply dimension of foreign fighter activism remains poorly understood.

Islamist foreign fighter recruitment in the West has so far only been studied in depth by Ciluffo, Ranstorp, and Cozzens (2010). They muster important anecdotal evidence on how fighters are recruited, what they do while abroad, and how they are affected by the experience. However, they do not assess the scale of the phenomenon or its relationship with domestic militancy. In the following, I try to be more specific.

COUNTING DOMESTIC FIGHTERS

To describe the relative distribution of domestic and foreign fighters, we need some absolute numbers for the supply of each. This section explains how I estimated the number of domestic fighters between 1990 and 2010.

I built a new dataset on Islamist attack plots and plotters in the West between 1990 and 2010, entitled the “Jihadi Plots in the West (JPW) Dataset.” I started in 1990 because jihadi attacks in the West were rare before that time. I could not use established terrorism datasets (ITERATE, RAND/MIPT, and GTD) to estimate recruit supply, because they exclude foiled attacks and lack information on plot participants. More detailed incident overviews exist, but none covers the entire West for both the 1990s and 2000s. To construct the JPW Dataset, I collated incident data from existing overviews by other scholars: Nesser (2008a; 2010) for Europe; Benjamin and Simon (2002), Jenkins (2010), and Bjelopera and Randol (2010) for the United States; Mullins (2011) for Australia; and Egerton and Wilner (2009) for Canada.3

These overviews are well referenced and rarely disputed. Underreporting is conceivable, but unlikely; government concealment of foiled plots presumably only concerns a limited number of small conspiracies. Western governments gain from informing their publics about thwarted attacks, because it demonstrates competence and deters future attackers. Overreporting is a more serious concern, because the lists include a variety of incident types and require some sifting to identify confirmed domestic jihadi plots—those that (1) involved violence, (2) were intended for Western soil, (3) involved Islamists, and (4) were reasonably well documented. Jenkins (2010) and Bjelopera and Randol (2010) include cases of foreign fighting and fundraising (and even some plots by non-Muslims), so from these two overviews I included only incidents involving violence by Islamists on U.S. territory.4 Mullins (2011) includes cases of foreign fighting, so from his study I extracted only operations due to occur on Australian soil. Nesser (2008a; 2010) overreports in a different way: All his plots occur in Europe, but some are poorly documented. Recognizing this, Nesser codes for reliability using a three-tier system, from which I include only “category 1” and “category 2” incidents.5 The extraction yielded a total of 106 domestic attacks or plots: 68 in Europe, 32 in the United States, 1 in Canada, and 5 in Australia.

Clearly, coding foiled plots is problematic, because one can never be sure what the suspects’ intentions really were. There is a risk of including false positives, because media reporting often favors police allegations over a suspect's innocence claims. Moreover, there may be minor reporting biases stemming from variation in prosecution practices or media reporting constraints between countries. It could also be that the compilers of these overviews were unequally meticulous. All these problems notwithstanding, the dataset arguably represents our best estimate yet of the population of intended attacks in the West between 1990 and 2010.

Having identified the plots, I moved to the plotters, seeking to establish the number and identity of core participants. I placed the bar of inclusion fairly high, to include only people who were directly involved in operational matters. In most cases, core plotters were easy to identify, but in some instances identification required a subjective assessment based on poor or conflicting evidence.6 There may be some overreporting, because some cases involved many initial arrests and no follow-up information about the participants’ roles, forcing me to include all initial detainees. Individuals were only counted once; in the few cases where the same network perpetrated more than one attack (such as the GIA bombings in France in the late 1990s), attack number 2 and up were coded as having no perpetrators.

I then gathered biographical information on each plotter, seeking to establish whether they had previously fought abroad. This information enabled me to distinguish the “fresh” domestic fighter supply from the foreign-turned-domestic fighters. To obtain this information I relied on the earlier mentioned incident overviews, as well as these search engines: Lexis-Nexis, World News Connection, and Google. Past foreign fighter experience is likely to be underreported, especially for participants in the less well-documented European plots. This means that the “fresh” domestic fighter supply may be somewhat lower than my estimate suggests.

Finally, I collected evidence on declared intentions, using the same types of open sources. First, for the foreign-turned-domestic fighters, I looked for the declared purpose of the initial trip abroad, coding three types of motivations: to fight abroad, to train abroad for a domestic attack, or unknown. Second, for other domestic fighters, I looked for evidence indicating whether they had expressed a desire to travel abroad as foreign fighters before engaging in the domestic attack. These data become relevant later when I try to explain the relative distribution of domestic and foreign fighters.

Overall, I count 401 plotters, of whom at least 107 were previously foreign fighters. Thus 294—or let us say roughly 300—is my best estimate for the aggregate fresh supply of domestic fighters (see Table 1). This represents the estimated total number of Islamists in the West who, in the period between 1990 and 2010, were prepared to proceed directly to domestic attacks without going abroad first.

Estimated supply of Islamist domestic fighters in the West, 1990–2010, based on JPW dataset

TABLE 1.

Estimated supply of Islamist domestic fighters in the West, 1990–2010, based on JPW dataset

*A “veteran” is a domestic fighter with prior foreign fighter experience.

These estimates are, of course, tentative and should be read as indicators of order of magnitude, not precise measurements.

COUNTING FOREIGN FIGHTERS

This section explains how I estimated the supply of foreign fighters for the same period. This is a tricky task, because reliable data are elusive. I concur with Egerton (2011: 119–20) that precise quantification is impossible, but I propose a way to generate rough estimates for comparative purposes.

Unfortunately, most estimation strategies are dead ends. It is not enough to count the number of foreign fighter veterans in domestic plots, because this number represents only the men who returned.7 Nor can we simply take data from the receiving end of the foreign fighter pipeline (i.e., from lists of foreign fighters in Iraq and elsewhere) because such lists are few and unreliable.8 Finally, relying on reports of people arrested or indicted for foreign fighting is equally problematic due to geographical and chronological bias in the reporting.9

My solution was to aggregate anecdotal evidence in the form of numerical estimates of foreign fighter flows provided by qualified observers in written sources. To construct the observation set, I first compiled a bibliography of the bodies of secondary literature most likely to contain such observations, namely, the literature on individual conflicts known to have attracted foreign fighters and the literature on al-Qaida and transnational jihadism. I also compiled a list of primary sources that might contain such numbers, notably memoirs by former foreign fighters (e.g., Collins 2002; Nasiri 2006). Finally I gathered news reports from Lexis-Nexis with relevant search terms (“<destination country> AND ‘foreign fighters,’” “<destination country> AND ‘foreign volunteers,’” “<Source country> AND ‘foreign fighters,’” “<source country> AND volunteers AND jihad”). I then skimmed all the material, recording every reference to foreign fighter movements that (1) contained a specific number and (2) clearly referred to Western-origin volunteers. This generated a list of 55 observations. Most are in the form of statements by Western intelligence officials, Islamists, journalists, or academics. Veteran foreign fighters from the JPW dataset were not included as observations. I sorted the observations by departure region and put them in rough chronological order. The data were not processed or coded any further due to their heterogeneous nature. Instead I used them to subjectively generate rough estimates.

This approach is fraught with problems. For one, the observation set is undoubtedly incomplete, because I cannot claim to have exhausted the literature. Second, individual observations are highly unreliable, because many actors have an interest or inclination to inflate or deflate numbers. Third, there are reporting biases, with some conflicts and some periods better covered than others. Fourth, there is unit heterogeneity: Some observations contain just a few names, others offer snapshot estimates from a single point in time and space, whereas yet others constitute aggregate estimates covering several years and conflicts. Still, as argued later, these are the least bad data available, and they provide our only way of getting at least a rough sense of the scale of the foreign fighter phenomenon.

To avoid confirmation bias in favor of the argument I propose, I extracted only a very conservative estimate from the 55 observations. The subjective estimation process can be summarized as follows (see also Table 2):

  • • For the United States in the 1990s, an FBI official speaks of 1,000–2,000 American foreign fighters (observation #2), a journalist refers to three dozen named individuals (observation #3), and two academics speak of “hundreds if not thousands” (observation #4). There are additional reports of small groups in Bosnia and elsewhere (observations #5–15). Let me be very conservative and say 100.
  • • For the United States in the 2000s, reports indicate “at least 20” in Somalia (observation #19) and a few people in Yemen (observation #20). In addition, Jenkins and Bjelopera and Randol together report 82 named individuals (observations #17 and #18).10 A conservative estimate is therefore 100.
  • • For Europe in the 1990s, there is an outlier report of 1,000–2,000 from the whole continent (observation #25) and another of “several hundred” (observation #26), whereas observation #27 speaks of “several hundred” Europeans in Bosnia alone. In addition there are country-specific reports, for example, 400 French in Afghanistan (observation #24) and 50 French in Bosnia (observation #31). A conservative estimate is thus 200.
  • • For Europe in the 2000s, observations #36–52 converge on at least 100 for Iraq, at least 200 for Afghanistan/Pakistan, at least 10 in Yemen, and at least 150 in Somalia. A rough conservative estimate is thus 500.
  • • For Australia, we have two observations from the 2000s: the 14 named individuals (observation #53) and a report of 10–40 Australians in Somalia in the 2000s (observation #54). Let us use an estimate of 25.
  • • For Canada, we have only one observation (#55) of approximately 20 people in Somalia in the 2000s. Let us say 20.
Estimates of the supply of Islamist foreign fighters from the West, 1990–2010, based on observation set

TABLE 2.

Estimates of the supply of Islamist foreign fighters from the West, 1990–2010, based on observation set

Thus my best minimal estimate of the supply of foreign fighters from the West between 1990 and 2010 is 945 individuals. Let us say “around a thousand” for convenience. Notethat this is a very conservative estimate. The same observation set can arguably justify a maximum estimate of 7,500 individuals (i.e., more than 350 per year).

COMPARING THE NUMBERS

A comparison of aggregate numbers thus suggests that the supply of foreign fighters has been larger than that of domestic fighters at a ratio of at least 3:1 (945 versus 294). In reality, the distribution may be even more skewed, given that the maximum estimate of foreign fighters is 25 times that of domestic fighters.

Of course, these aggregate estimates are quite unreliable. One way of testing the robustness of the ratio is to consider a set of supply estimates from a smaller, but better documented period and area, thus trading validity for reliability. A good candidate is the United States in the 2000s. Jenkins (2010) and Bjelopera and Randol (2010) report 24 cases of foreign fighter activism involving a total of 82 individuals. By comparison, the estimated number of domestic attackers in the same country and period in the JPW dataset is 58, only 42 of whom did not have previous foreign fighter experience. These are also just estimates, but presumably more reliable than the global ones presented earlier. Here too, foreign fighters outnumber domestic fighters by two to one.

Supply rates almost certainly vary over time and between countries. Unfortunately, the data are too patchy to allow for detailed (country-year) disaggregation, but we can do it by decade and continent (excluding Canada and Australia for a lack of data). Of course, disaggregating already tentative estimates is problematic, but the rough proportions of the variation are noteworthy (see Table 3).

Estimated supply rates and ratios of Islamist domestic and foreign fighters in Europe and the United States before and after September 11, 2001

TABLE 3.

Estimated supply rates and ratios of Islamist domestic and foreign fighters in Europe and the United States before and after September 11, 2001

In absolute terms, the supply of both domestic and foreign fighters appears to have increased from the 1990s to the 2000s. Interestingly, most of the increase seems to have occurred in Europe, not in the United States, where the combined domestic-foreign fighter supply stayed roughly the same. Most interesting, however, are the relative changes. The supply of domestic fighters appears to have increased much more than that of foreign fighters in both the United States and Europe. This produces a striking decrease in the foreign-to-domestic-fighter ratio over time, from 11:1 in the 1990s to 3:1 in the 2000s. Thus, although foreign fighting seems always to have been the more popular choice for Western jihadists, it became relatively less popular in the 2000s. I offer possible explanations for this change in a later section.

These variations notwithstanding, the key finding from the tentative data is that, throughout the period from 1990 to 2010, the supply of Islamist foreign fighters in the West appears to have been larger than that of Islamist domestic fighters. Put differently, when Western jihadists first considered using violence, they were, all else equal, more likely to join a distant war zone than attack at home.

EXPLAINING THE DISTRIBUTION

This section considers three candidate explanations for why foreign fighting has proved the more frequent choice of Western jihadists: (1) because it is easier, 2) because they need training, and (3) because they prefer it.

Opportunity

The first hypothesis suggests that militants go where it is easier to operate. It may be that choices are informed by domestic constraints (i.e., that the preemptive capability of Western security agencies on home turf is so high that Western jihadists go abroad instead). This hypothesis rests on the underlying assumption that the individual does not care much where he engages.

To probe the plausibility of this hypothesis, we can ask whether it makes sense to argue that it is much easier for a Western jihadist to operate abroad than at home. Much suggests that it does not. Planning an attack in the West is risky, but so is foreign fighting; it involves crossing international borders, navigating unfamiliar territory, dodging heavy-handed local police, and dealing with unknown new contacts. There is a considerable risk of being arrested, tortured, wounded, or killed in the search for access to a conflict zone.11 Although domestic counterterrorism measures have proliferated post-9/11, the same is true at the international level. States share more intelligence and cooperate more on counterterrorism. International travel is better monitored and ID papers harder to forge. Western states take more direct action, in the form of espionage, special operations, drone attacks, and the like, to dismantle jihadi networks in the Muslim world. One might even argue that domestic fighting can be easier than foreign fighting, because there is anecdotal evidence of jihadists who attacked at home after finding foreign fighting impractical.

The picture changes if we assume that jihadists care about the duration of the fighting experience. A key difference between domestic and foreign fighting is that in the West, attackers rarely get more than “one shot” before being killed or arrested, whereas they can fight longer if they reach a conflict zone. There is anecdotal evidence of a “Hemingway effect” associated with foreign fighting; that is, some recruits are motivated by the search for adventure (Sageman 2004). For these individuals, duration presumably matters more, making opportunity a major factor informing theirtheater choice. However, we do not know the proportion of foreign fighters for whom this is the case, because adventurism is not a motivation that activists usually declare.

Another indication that countermeasures matter is the chronological evolution of the foreign fighter-domestic fighter ratio. The opportunity hypothesis implies that the relative prevalence of foreign fighting is correlated with the level of constraints on domestic fighting relative to that on foreign fighting. It is reasonable to assume that, after 9/11, international counterterrorism improved relatively more than domestic counterterrorism because the improvement potential was larger. We should thus expect to see the relative popularity of foreign fighting decline after 9/11, which is exactly what we did see. However, higher constraints on foreign fighting are likely not the whole story; shifting preferences may also have something to do with the ratio change.

In sum, physical obstruction may account for some of the variance in theater choices, but it is likely not the most important factor.

Training

A second hypothesis is that recruits prefer operating in the West, but train abroad first to increase capability. Here, foreign fighting is an instrumental strategy in which militants forsake short-term tactical opportunities for the deferred gratification of a more destructive operation in the future. This view is common among analysts and policy makers today, and it underpins the widespread assumption that foreign fighters are domestic terrorists in the making.12

It is true that good fighting skills require physical practice and that military know-how is more widely available in the conflict zones of the Muslim world than in the West. Moreover, the data presented later in this article suggest that foreign fighters really do make more lethal domestic operatives. However, does it make sense to argue that training abroad is necessary for a domestic operation? Probably not. For one, a domestic attack need not be very destructive or complicated, as al-Qaida itself has told its followers (see the later discussion). Second, certain forms of military instruction are available in the West, either offline from experienced operatives or online from instruction manuals. Third, the instruction offered abroad is not necessarily of the type required for attacks at home. Testimonies by foreign fighters (al-Amriiki 2012; Collins 2002; Nasiri 2006) indicate that most recruits receive only basic paramilitary training while abroad; only a handful learn bomb making, countersurveillance, and other skills needed to operate in Western cities. Fourth, the benefits of training abroad are counterbalanced by substantial costs, in the form of risks associated with foreign fighting.

Perhaps for these reasons, several prominent jihadi leaders and ideologues have actively discouraged foreign fighting.13 Interestingly, the discouragement has tended to come from precisely those ideologues who most vehemently advocate operations in the West. Abu Mus'ab al-Suri, a prominent pro-al-Qaida ideologue, cautioned in 2004 against training on what he called “open fronts” (i.e., the major war zones in the Muslim world), because foreign travel is costly and risky (Lia 2008: 534–35). More recently, the English-language jihadi magazine Inspire repeatedly encouraged prospective militants in the West to act at home and not to go abroad for training (Ibrahim 2010: 55). In 2010 and 2011, the prominent U.S-Yemeni ideologue Anwar al-Awlaki, then based in Yemen, persuaded several Western recruits via e-mail to act at home rather than abroad; in one encrypted message, he told the UK-based Rajib Karim, “[Hearing] that you intend on making hijrah [emigration], I immediately wanted to contact you to tell you that my advice to you was to remain in your current position” (Berger 2011). Even some senior leaders in al-Qaida Central in Pakistan have encouraged Western Muslims to engage in “individual jihad” at home. For example, in 2011, Adam Gadahn, the American convert-turned-al-Qaida spokesman, said,

[Western Muslims are] perfectly placed to play a . . .part in the jihad . . . as an example, America is absolutely awash with easily attainable firearms. You can go down to a gun show at the local convention center and come away with a fully automatic assault rifle without any background check. . . . So, what are you waiting for? (Brachman 2011).

Two other indicators further undermine the training hypothesis. One is the proportion of foreign fighters who actually return. My tentative data indicate that only about one in nine foreign fighters returned for an attack in the West (see the later section on “estimating the veteran effect”). If training was the main purpose of travel, we would expect more people to come back. Death or capture abroad might explain some, but far from all, of the missing returnees.

A second indicator is the foreign fighters’ declared initial intentions for going abroad. In the biographies of the 107 foreign-fighters-turned-domestic-fighters in the JPW dataset—the subset of foreign fighters most likely to have intended to eventually return—I found only 2 individuals who explicitly stated they went abroad to train for a future domestic attack. The likelihood of underreporting prevents us from making too much of this observation, but it is worth noting, especially in light of the fact that the same sample contains 45 cases of individuals declaring that the initial purpose of the trip was to join a foreign insurgency (see Table 4).

Declared initial purpose of first trip abroad by foreign-turned-domestic fighters

TABLE 4.

Declared initial purpose of first trip abroad by foreign-turned-domestic fighters

The available evidence thus stacks up against the training hypothesis. Some foreign fighters may well have left in search of training for a domestic attack, but the need for training does not plausibly account for the full extent of the popularity of foreign fighting.

Norms

A third hypothesis is that militants prefer going abroad because they view foreign fighting as more legitimate than domestic fighting. In this hypothesis, involvement by Western Islamists in militancy depends on perceived religious sanction, and the predominant view among Islamist religious authorities is that fighting in established conflict zones is more legitimate than attacks in the West. Given the distribution of views of religious authorities, most Western jihadis find fighting in established conflict zones the legitimate avenue of their new beliefs. Note that this is not an argument about a static ideology determining the behavior of its followers. The strength of the norm against out-of-area attacks may vary in time and space, and a given individual's perception of the same norms is sensitive to personal disposition and context.

Let us begin by asking what Islamist religious authorities say about where war can or should be waged. Although no codified Islamic laws of war exist, there are views on the conduct of war that can be considered mainstream. The legal tradition notably reflects norms of proportionality and discrimination: One should not cause unnecessary death or destruction, and one should not target noncombatants (Kelsay 2007: 97–124; Peters 1996: 145–47). Moreover, in the theological literature, the default understanding of military jihad is one in which two conventional armies—one Muslim, the other non-Muslim—fight each other on a contiguous piece of territory (Cook 2005: 122–23). Differences of clerical opinion arise mainly on dilemmas confronting regular armies, such as whether to shell enemy units that use human shields or how to treat prisoners of war (Aboul-Enein and Zuhur 2004; Morabia 1993: 227–30). Throughout the ages, Islamic scholars have always declared certain forms of political violence as illegitimate, especially those that deliberately target civilians (Abou el Fadl 2001). In the modern era, mainstream Islamic scholars have repeatedly condemned terrorism (irhab; Jackson 2001; Misri 2005; Schwartz 1991).14 Of course, norms against terrorism are diluted by definitional problems; certain activities considered terrorism by Western governments are viewed as legitimate resistance by some Islamic clerics (Kelsay 2007: 140–41). Generally speaking, however, the more conventional a military technology, the more legitimate it is considered in Islamic law. When waged by a Muslim rebel group on home territory against a non-Muslim army, insurgency is thus considered more legitimate than international terrorism.

However, as with many issues in Sunni Islam there exists a range of legal opinion. At the radical end of the spectrum, some clerics, such as the Saudis Hamud al-Shu' aybi, Nasir al-Fahd, and Abd al-Rahman al-Jarbu‘, have declared mass casualty operations in the West to be entirely legitimate (Hegghammer 2010). These pronouncements are in addition to the many lay activist-ideologues linked to al-Qaida, such as Usama bin Ladin (Lawrence 2005), Abu Mus'ab al-Suri (Lia 2007), and Anwar al-Awlaqi (al-Malahim 2010). Ideologues who condone attacks in the West typically stress three arguments: (1) the Prophet Muhammad allowed, in certain circumstances, for the killing of civilians and for collective punishment; (2) civilians in democracies are responsible for their governments’ policies; and (3) non-Muslims are killing Muslim civilians, so Muslims can reciprocate. Mainstream clerics reject this position, typically by arguing that (1) the situation of Muslims today is not analogous to that of the Prophet; (2) the intentional targeting of civilians, as opposed to unintentional collateral damage, is categorically forbidden; and (3) violence in the West is not permitted in the first place due to the existence of international treaties (March 2011).

Even at the radical end of the spectrum of clerical opinion, there is some debate about attacks in the West, whereas almost no clerics question the legitimacy of geographically limited insurgency. For example, Abdallah Azzam, the influential Palestinian cleric described as “the mentor of Usama bin Ladin,” never called for nor organized any out-of-theater attacks during his eight years of fighting the Soviets in Afghanistan in the 1980s (Hegghammer 2011). More recently, the UK-based Syrian ideologue Abu Basir al-Tartusi surprised many of his jihadi followers in 2005 by publicly condemning the London bombings (Al-Tartusi 2005; Paz 2005b). According to Nesser (2011a: 181), even the most fiery jihad clerics in Europe in the late 1990s and early 2000s, such as Abu Qatada al-Filastini, Abu Hamza al-Masri, and Umar Bakri Muhammad, “seem to have had certain reservations about launching terrorist attacks in Western countries.” Nesser (185) adds that “although Abu Hamza justified violent jihad in any country, including non-Muslim host states, he remained mainly focused on supporting insurgencies and liberation wars in Yemen, Chechnya, and Kashmir until he was arrested in 2004.”

It is, of course, difficult to measure with precision the relative prevalence and influence of different ideological positions. Relying on behavioral indicators brings endogeneity problems, and quantitative content analysis is difficult because of the vastness of the universe of texts. Because of the lack of better methods, we can rely on qualitative assessments by academic specialists on religious literature (Bonner 2006; Cook 2005; Firestone 1999; Khadduri 1966; Sivan 1998), virtually all of whom maintain that most Muslim clerics condemn out-of-theater attacks on non-combatants. It follows that clerics who approve of foreign fighting most likely outnumber those condoning domestic fighting.

This distribution of views appears to be reflected in the broader Muslim population. Unfortunately, no major surveys explicitly ask about views on the relative legitimacy of terrorism and insurgency. However, a few surveys distinguish between support for al-Qaida and support for local insurgent groups such as Hamas and Hizballah (Pew 2010), and the results suggest that the latter two enjoy considerably more support than the former among non-Palestinian and non-Lebanese Muslims. Similar findings appear in qualitative surveys of young Muslims in the West. A study based on interviews with a cross-section of 70 young Muslims in Canada concludes that “it is possible—and common—to believe violent jihad in Western countries is unacceptable, while simultaneously believing violent jihad in Muslim majority countries is acceptable” (Bartlett, Birdwell, and King 2010: 96). Another study based on semi-structured interviews with Islamists in Europe reports widespread support for Muslim insurgents on the one hand and skepticism toward violence in the West on the other (Kühle and Lindekilde 2010: 55–58). The general attitude is reflected in the following statement by a young British Muslim interviewed by Neumann and Rogers (2007: 79):

I understand the acts of resistance in Afghanistan. They are freedom fighters. They didn't go to the United States to carry out acts of terrorism. I sympathise with the people who fight in Afghanistan, in Iraq, in Palestine, in Sudan. I don't agree with the ones who commit acts in an underground, a train, a plane.

Social desirability bias is obviously a concern in this type of data, but the bias should not be overestimated for several reasons. Some informants in the same study did admit to condoning attacks in the West. Moreover, support for violence against Western troops abroad is also considered controversial by mainstream Western society, yet most informants declared their support for it. Third, some Islamists have expressed similar views while not in an interview situation. For example, Karim Bourti, a French Algerian actively involved in recruitment to Chechnya and Afghanistan, was captured on hidden camera in 2002 saying that “those who decided [in the mid-1990s] to strike France had made a mistake both strategically and from the point of view of dogma” (Sifaoui 2003: 53).

What about militants? The proposition that Western jihadists heed norms when deciding where to fight can be assessed by looking at declared motivations and motivation sequencing. If most foreign fighters say they intend to join insurgencies abroad and most militants acquire the motivation to fight abroad before the motivation to attack at home, then the norms hypothesis is strengthened.

Measuring declared intentions is, of course, problematic. We often lack reports of intentions, and when we do have them, there is no guarantee that the source is being truthful. That said, it would be unreasonable to dismiss declared intentions as completely irrelevant. Consider again the 107 foreign-fighters-turned-domestic-fighters in the JPW dataset. Their subsequent involvement in domestic operations makes them the least likely of all foreign fighters to have considered insurgency as the end purpose of their departure abroad. Yet, at least 45 of these 107 appear to have originally wanted to join insurgencies, and not train for domestic operations (see Table 4). Examples include Najibullah Zazi (2009 New York subway plot), who said he went to Pakistan with the intention of joining the Taliban in Afghanistan (Sulzberger and Rashbaum 2010); Mohammed Siddique Khan (2005 London bombing) who “originally . . . wanted to join Taliban's war in Afghanistan (Silber and Bhatt 2007: 48), and Fritz Gelowicz and Adem Yilmaz (2007 Islamic Jihad Union plot, Germany) who “told the court that they originally wished to join the Sunni resistance in Iraq” (Nesser 2011b: 474).

As far as motivation sequencing is concerned, we already know that it is more common to fight abroad before fighting at home than the other way around, and that very few foreign fighters expressed the intention to return for domestic operations before they went abroad. What about those who attacked at home without going abroad first? A closer look at their biographies reveals that several (at least 13 of 294) expressed an intention to fight abroad before plotting domestic attacks. For example, in the late 2000s the so-called North Carolina cluster went abroad in search of combat experience; only when they failed to join a battlefront did they contemplate home operations (U.S. v Boyd et al. 2009). So did, allegedly, Tarek Mehanna, Ahmad Abousamra, and their unnamed associate from the “Mehanna case” (U.S. v Mehanna and Abousamra 2009). Two other U.S.-based militants, Farooque Ahmad and Mohammad Osman Mohamoud, both first expressed a desire to go abroad for jihad, but were drawn into domestic plots by FBI informers (U.S. v Ahmed 2010). For Europe we generally have less information about individual trajectories, but we know, for example, that Jamal Ahmidan from the 2004 Madrid bombing “spoke incessantly about jihad and his desire to fight the Americans in Iraq” (Silber and Bhatt 2007: 39) and that Samir Azzouz, a member of the Hofstad group, made a failed attempt to travel to Chechnya in 2002 (Crawford 2004).

Another point relevant to motivation sequencing is that the imagery used in typical al-Qaida recruitment videos is almost exclusively drawn from conflict zones, such as Palestine, Iraq, Afghanistan, Kashmir, Chechnya, Bosnia, and Somalia (Farwell 2010; Finsnes 2010; Salem, Reid, and Chen 2008). This suggests that in the early radicalization phase, recruits are exposed to more propaganda about foreign conflicts than about domestic operations. It also indicates that propagandists expect new recruits to be more sensitive to imagery from established war zones than to examples of operations in the West.

A final indicator of the impact of norms is the pre-attack deliberations of domestic fighters. Occasionally, scholars can observe these deliberations in transcripts of secret recordings or intercepted communication made public during trials. The limited evidence available shows that operatives often doubt, discuss, and consult others about the permissibility, in Islamic legal terms, of their intended actions. Nesser notes that, in the 2000s, “there have been recurring discussions about security treaties [i.e., Islamic legal opinions regulating the use of force by the Muslim community against non-Muslim states] on radical websites, and captured terrorists possessed texts dealing with the topic.” For example, in 2003, Jason Walters, a member of the Dutch cell known as the Hofstad group, consulted a certain Imam Abdul-Jabbar van de Ven about the legality of attacking in the West (Silber and Bhatt 2007: 57). Similarly, in 2009, a group of Somalis in Australia plotted an attack on a military facility in Australia.They were so unsure about the legitimacy of their operation that one cell member traveled all the way to Somalia to seek permission from a religious cleric (Munro 2010). Thus, even the most radical activists seem to realize that domestic fighting is considered illegitimate by many.

The norms hypothesis also plausibly accounts for at least some of the decrease in the foreign-to-domestic fighter ratio (from 11:1 in the 1990s to 3:1 in the 2000s). We know that the normative consensus against attacks in the West weakened in the Islamist community from the late 1990s onward (Gerges 2006). In the early 1990s, virtually no Islamist figure publicly articulated arguments for attacks in the West. It was not until 1998 that al-Qaida published the first in a series of statements advocating attacks on Western civilians worldwide (Lawrence 2005).15 After 9/11, however, many more ideologues and some religious clerics declared it legitimate to attack in the West. The 2000s saw an exponential growth in the amount of Internet propaganda advocating this position. It seems plausible that the weakening of the normsagainst domestic fighting negatively affected the foreign-to-domestic-fighter ratio. This explanation is not incompatible with the one about higher physical constraints on foreign fighting described earlier. Most likely, both factors contribute to the ratio decrease, although it is difficult to disentangle the precise effect of each. In addition, there may be an interaction effect between norms and physical constraints, namely, that clerical views are sensitive to changes in counterterrorism practices. We saw earlier that al-Qaida clerics discouraged foreign travel partly on the basis of its risks and that they discouraged it more in the 2000s when it had become more difficult.

In sum, the norms hypothesis emerges as the strongest explanation for the prevalence of foreign fighters. A majority of Western jihadis choose foreign fighting over domestic fighting, most likely because they have come to view the former as more legitimate after observing the distribution of views among religious authorities. The preference for foreign fighting among Western jihadists as a group has weakened over time, because foreign fighting has become more difficult and more ideologues encourage domestic fighting. Still, the preference remains strong.

ESTIMATING THE VETERAN EFFECT

Even though most foreign fighters appear not to leave with the intention of returning for domestic attacks, some clearly do acquire this motivation along the way. This section looks at how many they are, why they return, and how they affect violence in the West.

Return Rates

My data, with all its limitations, indicate that no more than one in nine foreign fighters returned to perpetrate attacks in the West (107 returnees against 945 foreign fighters). The return rate may well be even lower, because the foreign fighter estimate has a larger upside than that of returnees. On the one hand, this rate suggests that far from all foreign fighters are domestic fighters-in-the-making. On the other hand, a one-in-nine radicalization rate would make foreign fighter experience one of the strongest predictors of individual involvement in domestic operations that we know. The predictive power of other biographic variables—whether nationality, economic status, or any other biographical trait studied so far—does not come close (Bakker 2006; 2009; Egerton 2011; Sageman 2004; Venhaus 2010). Unfortunately, however, the question of differential radicalization—why only some foreign fighters become domestic fighters—cannot yet be answered for lack of data.

Radicalization Processes and Mechanisms

Why would any foreign fighter return for an attack if he originally decided it was preferable to leave? A full answer to this question is beyond the scope of this study, but I sketch out two hypotheses for further testing. These hypothetical processes were generated inductively from observation of known cases of foreign fighters “going domestic.” There may be additional processes, but the two I propose seem to be more common.

The first, called “enlistment,” refers to a trajectory in which the foreign fighter is drawn into domestic fighting by a calculating second party. His own initial intention was to join a foreign insurgency, but he finds himself in a training camp operated by a group with plans to attack in the West. The recruit may or may not be aware of the group's international ambitions when joining. Over time he develops a sense of loyalty to the leadership and makes friends with fellow trainees. At some point he is asked about his general willingness to serve the organization by participating in an operation, the details of which may or may not be known to him. Peer pressure, a sense of loyalty, and/or a lack of exit options make him accept. When the order to deploy to the West arrives, he is so deeply invested in the organization and has so few ways out that he complies, even if the operation is of a type that he originally considered illegitimate.

There are numerous examples of this process, the most famous being the so-called Hamburg Cell of the 9/11 plot, whose members went to Afghanistan with the intention to train for jihad in Chechnya (9/11 Commission 2004: 165), only to be drawn into the 9/11 plot by al-Qaida after arriving there. Another example is Najibullah Zazi (2009 New York subway plot), who told the court,

Our plan was to go to Afghanistan and fight with the Taliban. While we were in Peshawar, we were recruited by Al Qaeda, instead. We were taken by Al Qaeda to training camp in Waziristan, where we received weapons training. During the training, Al Qaeda leaders asked us to return to the United States and conduct martyrdom operation. We agreed to this plan. I did so because of my feelings about what the United States was doing in Afghanistan (New York Daily News 2010).

In the second process, which may be called “socialization,” the foreign fighter's preferences change through the experience of military life. Here, the recruit is pushed toward domestic fighting by a gradual change in perceptions on the legitimacy of domestic fighting; no second-party manipulation is involved. The recruit arrives in the conflict zone, takes part in combat, and comes to see theological arguments constraining violence as impractical or naïve. Having participated in insurgency, he comes to view domestic fighting as just a small and permissible escalation. He decides to return, inclined to operate in the West, but without a specific plot in mind.

One example of this pattern is provided by the so-called Chechen Network, which plotted attacks on the Russian embassy in Paris in 2002, having previously trained in Chechnya but without having forged ties with a particular organization (Nesser 2011b: 274ff). The same appears to have been the case with the Frankfurt cell, which plotted an attack on the Strasbourg Christmas market in 2000, after training in Afghanistan (Nesser 2011b: 223ff). More recently, the “Stockholm bomber” Taimour Abdelwahhab trained in Iraq before perpetrating a botched suicide operation in late 2010, apparently on his own initiative (Reuters 2011).

The precise mechanisms by which individuals come to defy normative constraints on terrorism in these two processes remain to be identified. One possible mechanism is continued exposure to selective information about clerical views on terrorist tactics. In the field, the foreign fighter is surrounded exclusively by fellow militants. Moreover, he is exposed to a narrow range of clerical opinion on violence, because such opinions only reach him through the host organization's own ideologues, literature selected by the host organization, or fellow recruits. This is in contrast to the home setting, where he is exposed to a broader range of opinion, for example in the local mosque or among family and friends. Moderate voices might not convince him that domestic attacks are categorically forbidden, but they introduce uncertainty about the validity of legal claims made by radical ideologues.

Both of these hypothetical radicalization processes require further testing through process tracing. All we know for now is that some foreign fighters acquire the motivation to attack in the West after going abroad.

Impact

The JPW data suggest that the presence of foreign fighter returnees increases the effectiveness of attacks in the West (see Table 5). Whereas only 26% of all plotters are known to have foreign fighter experience, around 46% of all plots (49 of 106) included at least one veteran. For executed attacks, the rate is 58% (14 of 24), and for executed attacks with fatalities, it is 67% (8 of 12). Twenty-nine percent of plots with veterans reached execution and 16% caused fatalities. For plots without veterans the corresponding rates are 18% and 7%. Here again, precise figures must be taken with a pinch of salt given the data problems, but the available numbers tentatively suggest that the presence of a veteran increases—by a factor of around 1.5—the probability that a plot will come to execution, and it doubles the likelihood that the plot will kill people.

Effect of veteran participation on domestic plot effectiveness

TABLE 5.

Effect of veteran participation on domestic plot effectiveness

* P refers to the probability that the plot will be executed or be lethal.

Other studies report similar results from different data. Sageman (2010) examined 60 plots in the West by 46 networks between 1993 and 2008, finding that 26 of the networks included a veteran and that a veteran doubled the probability of the attack reaching execution. Cruickshank (2010) examined the 21 most serious attack plots in the West between 2004 and 2010 and found that 12 plots involved a veteran and at least 37 of 131 plotters had been abroad. Gartenstein-Ross and Grossman (2009:14) noted that “over 40% of the sample traveled abroad for training or to fight jihad.” British authorities said in 2008 that three-quarters of post-9/11 UK plots can be “traced to Pakistan” (Coates and Page 2008). Clarke and Soria (2010: 28) noted that “seven of the eight major terrorist plots in the UK included in their cells one or more individuals who had attended terrorist training camps; six of the eight of these had been in Pakistan.”

My tentative data thus corroborate existing evidence of a “veteran effect” that makes returnees more lethal operatives. It is important, however, not to select on the dependent variable and infer that outgoing foreign fighters are more dangerous as a group, because as we know, most foreign fighters do not return for domestic attacks.

PROBING DIFFERENTIAL CHOICE

As noted in the introduction, the lack of demographics on the foreign fighter population precludes systematic investigation into the determinants of differential choice: why some stay while others leave. Nevertheless, in this section I cautiously probe this question, using available information and building on insights from the preceding analysis. The probe also serves to shed light on the limits of the norms hypothesis. Such limits clearly exist, because no less than 294 of 401 individuals in the JPW dataset attacked at home apparently without going abroad first.

For the sake of simplicity, I assume that the norms hypothesis is correct and that, all else equal, all first-time militants will prefer going abroad. Foreign fighting is thus the default choice, and variation stems from the unknown set of factors that make some first-time militants choose domestic attacks.

Because we cannot know whether a particular biographical characteristic—be it ethnicity, education level, or something else—predisposes agents to choose one or the other theater, our only hope is to identify process-based “treatments” correlated with specific choices. In the following, I discuss two such treatments: (1) personal contact with a foreign fighter veteran and (2) obstruction. I identified these two treatments inductively from the biographies of individuals in the JPW dataset who went straight to domestic fighting. I argue that these two factors, together with the norms hypothesis, plausibly account for a majority of choices. However, a residual category of unexplained decisions remains.

The most common factor appears to be veteran influence. For the reasons discussed in the previous section, veteran foreign fighters are more likely than nonveterans to view domestic operations as legitimate. They are familiar with the arguments used to counter violence-constraining legal claims, and their combat experience gives them authority and credibility among nonveterans.16 In addition, they may have developed other motivations, such as a taste of military life or a desire for vengeance. A veteran may thus radicalize the people around him by helping dispel doubts they may have about the legality of domestic attacks. This hypothesis is strengthened by the fact, noted earlier, that 49 of 106 plots involved at least one veteran and by anecdotal evidence that veterans often act as plot entrepreneurs.

A variant of this phenomenon may be agents provocateurs, that is, government informants who prod activists to participate in fictitious domestic plots, only to arrest them when they do so. This practice was particularly widespread in the United States in the 2000s, where government informants were involved in more than one-third of terrorism investigations (Kurzman, Schanzer, and Moosa 2011: 469) and in 62% of the prosecutions in the top 50 terrorism cases (Greenberg 2010: 20).17 In 28% of trials involving informants, the defendant claimed entrapment (Greenberg 2010: 20). This occurred in several cases in the JPW dataset, most famously in the Fort Dix, Riverdale Synagogue, and Herald Square cases. So far U.S. courts have dismissed all such claims, so we cannot say that entrapment in the legal sense has occurred. However, anecdotal evidence from several cases suggests that informants offered suggestions or subtle encouragement for an attack and in some cases provided weapons (CHRGJ 2011; Greenberg 2011). At least two ex-informants have admitted to offering such encouragement (Associated Press 2012; Harris 2012), and former FBI agents have criticized the practice (CHRGJ 2011: 48). The politicization of the entrapment debate and the lack of good data prohibit firm conclusions, but it is possible that agents provocateurs can play a role similar to that of veterans, by helping persuade otherwise hesitant subjects to act.

A second, less frequent mechanism may be obstruction. As mentioned earlier, at least 13 plot participants said they initially wanted to go abroad as foreign fighters, but found it difficult to do so and thus decided to act at home instead. This was notably the case with the Somali Australians in “Operation Neath” in 2009 (Munro 2010). There are many conceivable obstructing factors, including a lack of money or a passport, lack of contacts at relevant destinations, and a sense of being watched by the authorities. Obstruction helps individuals overcome normative concerns because it allows them to blame their involvement in the less legitimate violent activity on circumstance.

In the end, however, veteran influence and obstruction cannot account for all cases of people proceeding straight to domestic attacks. There is a non-negligible residual category of first-time militants who attack at home without knowing a veteran and without having been visibly obstructed. This residual category presumably comprises people making choices informed by unknown biographical or contextual factors. Until we obtain substantially better biographical data, the puzzle of differential choice unfortunately has to remain unsolved.

CONCLUSION

Western jihadists may not all be equally motivated to attack in the West. In fact, the tentative data presented here indicate that most prefer to fight outside the West and that most foreign fighters do not “come home to roost.” However, the data also point to a veteran effect that makes returnees significantly more effective operatives.

The article's main contribution is to offer baseline estimates of Western jihadists’ theater choices and some evidence on the motivations underlying them. The findings contribute to a growing body of scholarship on Western jihadism, specifically to a line of research that disaggregates the behaviors and motivations of militant Islamists (Moghadam and Fishman 2011). The article also adds to the literature on foreign fighters by empirically exploring the conceptual distinction between foreign fighting and terrorism (Colgan and Hegghammer 2011; Malet 2009).

The distinction revealed important insights, but must not be interpreted too rigorously. I am not suggesting that Western jihadists are divided into two distinct camps, that no foreign fighters leave for training, or that foreign fighters are harmless. Motivations can be fickle at the individual level, some militants do go abroad intending to return, and those who do are dangerous indeed. In addition, norms are malleable, so supply rates and ratios may change in the future. Because my findings pertain to past choices of jihadists as a group, they may be used to calibrate broad policy strategies, not to predict individual behavior.

Moreover, my findings are only valid for jihadists in the West. There is some evidence of a preference for foreign fighting among Islamists in Muslim countries (Hegghammer 2010), but more research is needed to confirm it. Of course, in established conflict zones, such as Iraq or Palestine, the situation is different, because Islamists there need not leave to wage insurgency. Looking beyond the Muslim world, the scope of my argument is limited by the scarcity of non-Islamist militants who regularly operate both at home and abroad. Of course, non-Islamist foreign fighters exist, but they tend to specialize in war volunteering and focus on a single conflict. For most recruits to the “International Brigades” in the Spanish Civil War, for example, operating at home was not an option. The same is true today of the young European leftists fighting with the FARC in Colombia (El Tiempo 2011). There are some exceptions; in the 1970s, for example, members of European extreme-left groups traveled to the Middle East to train with Palestinian guerrillas (Cooley 1973: 184–91; Karmon 2005) before returning to domestic operations. For the most part, however, inter-theater mobility of this kind has been unusual.

I anticipate at least two objections to my analysis. Some will argue that the data are too unreliable to support my conclusions. Many aspects of the coding of the JPW dataset can be discussed, and the basis for the foreign fighter estimates is very weak by the standards of modern social science. In response, I argue that good data on jihadism are notoriously difficult to obtain and that this study was a modest and transparent attempt to make the most of the available sources. Partial data are not always better than no data at all, but in this particular case I believe they are, because policies are already being informed by empirically unfounded assumptions about the threat posed by foreign fighters.

A second valid objection is that that the category “foreign fighter” itself is overaggregated. For example, there is a difference between fighting in one's country of origin (e.g., Somali-Americans going to Somalia) and fighting in a third country (e.g., Somali-Americans going to Afghanistan). There is also a difference between fighting one's host country (e.g., U.S. citizens fighting U.S. soldiers in Iraq) and fighting a third country (e.g., U.S. citizens fighting Indian soldiers in Kashmir). Finally there is the difference between leaving to train with international terrorists and leaving to fight with local insurgents. These distinctions are significant, but their exploration exceeded the scope of the article.

Indeed, the article has identified several questions for further research. Why do some militants proceed straight to domestic fighting without the “treatment” of foreign travel or veteran contact? Why do supply rates and ratios vary between countries and over time? Why do some destinations turn more foreign fighters into domestic fighters than others? Why do only some foreign fighters move on to domestic fighting? Do different types of foreign fighters (e.g., co-ethnic vs. non-co-ethnic ones) display different propensities to become domestic fighters?

My findings have several policy implications. First, to accurately measure domestic terrorist activity, analysts should distinguish more systematically between violence at home and abroad and consider abandoning the fuzzy term “homegrown terrorism.” Second, policy makers should distinguish between outgoing and returning foreign fighters and treat the latter as more of a threat. Prosecuting all aspiring foreign fighters as prospective domestic terrorists has limited preventive benefits, because so few of them, statistically speaking, will go on to attack the homeland. By the same logic, the use of agents provocateurs to draw aspiring foreign fighters into fake domestic plots may have limited preventive value. By contrast, returning foreign fighters and their contacts should be monitored very carefully. Third, governments should adapt their communication strategies to the reality that most Islamists consider confined insurgency more legitimate than international terrorism. Talk of insurgents and foreign fighters as “terrorists” will likely fall on deaf ears and may irritate rather than dissuade the fence-sitters. It is probably better to acknowledge the difference between domestic and foreign fighting and to discourage each activity with different sets of arguments.

For the social sciences more broadly, the article has illustrated the value of disaggregating phenomena that appear to be ideologically connected. The argument itself is not new; Kalyvas (2003) highlighted the problem of overaggregated variables in the study of civil war. Still, the view that radical Islamists are all the same has proved remarkably resilient. I hope that this article will help question that assumption.

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Notes

1 I define Islamism as “activism justified with primary reference to Islam” and jihadism as “violent Islamism.” I use “jihadist,” “violent Islamist,” and “militant Islamist” interchangeably.

2 I knowingly depart from existing definitions of “foreign fighter” (Hegghammer 2011; Malet 2009; Mendelsohn 2011), which often specify that foreign fighters engage in insurgency (not terrorism) and that they lack kinship ties to the insurgents they join.

3 These overviews were chosen for their thick plot description and inclusion of bibliographic references. Alternative overviews tend to include fewer details and references; see, e.g., Schanzer et al. (2010), Bakker (2011), Beutel (2011), Dahl (2011), Egerton (2011), Kurzman (2011), and New America Foundation (2011). Sageman's (2010) dataset is not public. Mueller and Stewart's (2012) overview of U.S. plots is detailed, but appeared too late for inclusion in this study. Although these incident lists may include somewhat more or fewer cases than those used for the JPW dataset, the differences are not substantial.

4 I thus excluded foreign fighter cases, plots involving only non-Muslims (e.g., Christopher Paul, Michael Reynolds, Ronald Grecula), attacks outside the West (e.g., Hasan Akbar's shooting of fellow U.S. soldiers in Kuwait), and cases of financial or logistical support to terrorist groups abroad (e.g., Syed Hashmi, who funded al-Qaida, and Hassan Abujihaad, who leaked information on U.S. Navy vessel locations).

5 In Nesser's (2008a: 926) classification, a category 1 incident requires “the existence of hard evidence that a terrorist attack was planned, prepared, or launched; that an attack struck a specific target or that a target or a type of target had been identified by the terrorists; and, finally, that clearly identifiable jihadis were behind the planning and attacks.” Category 2 includes incidents “in which information about suspected terrorists, targets, and intentions is vaguer, and which the author has not yet been able to verify or adequately substantiate through independent sources.” Category 3 includes events about which “information is very limited and vague.” The JPW dataset excludes two of Nesser's category 2 incidents, which I deem to be insufficiently documented.

6 For example, the 2004 Madrid bombing involved more than 20 people in various functions, but because the precise role of most of the participants remains unclear, I chose to include only the 8 people who barricaded themselves in the apartment in Lleganes after the bombing, plus Robei Osman Ahmed and Amer Azizi, who were not in Lleganes but are widely viewed as key co-conspirators (Reinares 2010).

7 Practically all Western jihadists are male. A few Islamist women from the West have performed support tasks for militants or accompanied their husbands in the field, but to this author's knowledge, nobody has taken part in combat.

8 Hegghammer (2011) provides estimates of foreign fighter contingent size in several conflicts, but not on contingent composition. It is only for Iraq that we have primary source data on foreign fighters data on foreign fighters broken down by nationality, but these data are too problematic to allow for extrapolation of overall estimates. For one, ambiguity regarding national identity means that, for example, a French Algerian could well be recorded in these sources as Algerian. Moreover, all samples have significant biases. The “Sinjar records” (Felter and Fishman 2007) cover only one organization's 12-month recruit supply. Other samples include only foreign fighters who were either killed (Paz 2005a), arrested (Krueger 2007), or suicide bombers (Hafez 2007). Obaid and Cordesman (2005) provide data, but no references. For what it is worth, the proportion of Western Islamists in these sources ranges from 1 in 154 (0.6%) in Paz (2005a), 6 of 606 (1%) in the Sinjar records, 6 of 331 (1.8%) in Krueger (2007), to 15 of 102 (14.7%) in Hafez (2007). Note that Iraq is by far the best documented foreign fighter destination; for other conflicts, data are much patchier.

9 European countries appear to have prosecuted foreign fighting less aggressively than the United States, and no Western government tracked foreign fighters systematically as a group before 9/11 (Kaplan 2002).

10 Jenkins (2010) and Bjelopera and Randol (2010) report a total of 107 foreign fighters, but 25 individuals appear in both counts.

11 No data exist on foreign fighter death rates. Anecdotal evidence (Hegghammer 2011:63) suggests they can vary from under 5% (1980s Afghanistan) to more than 90% (late 1990s Chechnya).

12 This observation is based on my many conversations with analysts, policy makers, and scholars in Europe and the United States over the past few years. My experience suggests the view is more widespread in the United States than in Europe.

13 There is admittedly some indirect evidence of the opposite; that is, jihadi leaders privately encouraging foreign training explicitly as preparation for domestic operations (Nesser 2008b: 239), but it is rare. More frequent are general exhortations for training abroad (Lia 2008), but they are usually not rationalized as preparation for domestic attacks.

14 For an overview of post-9/11 statements by leading Islamic scholars condemning terrorism, see http://www.unc.edu/~kurzman/terror.htm (accessed April 20, 2012).

15 In 1996 and 1997, Usama bin Ladin called for attacks on U.S. targets in the Middle East, not for operations in the United States (Lawrence 2005).

16 A new member of a jihadi network in France described the influence of veterans as follows: “Among my new brothers, there was a hierarchy based on one's involvement in the Muslim cause. Those who had done great and beautiful things were our leaders. We admired them; for us they were heroes” (Guendouz 2002: 110).

17 Similarly, Mueller and Stewart (2012: 84–87) code 24 of the 50 U.S. cases in their dataset as having been “essentially created or facilitated in a major way by the authorities.”