We have recently witnessed the fiftieth anniversary of the bombing of Hiroshima and Nagasaki, the only use of nuclear weapons in warfare. The non-use of nuclear weapons since then remains the single most important phenomenon of the nuclear age. Yet we still lack a full understanding of how this tradition arose and is maintained and of its prospects for the future. The widely cited explanation is deterrence, but this account is either wrong or incomplete. Although an element of sheer luck no doubt has played a part in this fortuitous outcome, this article argues that a normative element must be taken into account in explaining why nuclear weapons have not been used since 1945. A normative prohibition on nuclear use has developed in the global system, which, although not (yet) a fully robust norm, has stigmatized nuclear weapons as unacceptable weapons of mass destruction. Without this normative stigma, there might have been more “use.” This article examines this phenomenon in the context of the nuclear experience of the United States.

In this article I explore the propensity of democracies to use military force when involved in international disputes. I argue that the use of force by democracies in large part results from the domestic circumstances confronting their chief executives and that those circumstances vary predictably across democracies based on the structure of their domestic institutions. For example, U.S. presidents must garner public support before elections and maintain widespread congressional support if they involve the country in long-term military conflicts. Conflicts are risky without either of these domestic prerequisites. Consider President Lyndon Johnson's decision to escalate U.S. involvement in Vietnam. Johnson and his advisors reached their decision in 1964 but waited until early 1965, after the November 1964 national election, to announce this decision publicly and implement it on the ground. President George Bush's 1990 decision to double the number of troops deployed to Kuwait and move from deterring an attack on Saudi Arabia to compelling an Iraqi withdrawal was made in a similar manner. The Bush administration decided on its new policy in early October but announced that decision only after Congress recessed and the midterm elections were completed.

The end of the Cold War has profoundly transformed Europe's security situation. Although traditional security issues remain important, the most immediate threats to security since 1989 have originated not from relations between states, but from instability and conflict within states that has threatened to spill over into the interstate arena. States' efforts to shape and control this new security environment have resulted in a unique hybrid arrangement containing elements of traditional alliances, great power concerts, state and community building, and collective security.

Because of the nature of modern weapons, significant innovations in arms technology have the potential to induce dramatic changes in the international distribution of power. Consider, for example, the “strategic defense initiative” (SDI), a program initiated by the United States in the early 1980s. Had the program been successfully completed, it might have led to a substantial devaluation of Soviet nuclear capabilities and put the United States in a very dominant position. It should not then come as a surprise that interstate rivalry, especially among super powers, often takes the form of a race for technological superiority. Mary Acland-Hood claims that although the United States and the Soviet Union together accounted for roughly half of the world's military expenditures in the early 1980s, their share of world military research and development (R&D) expenditures was about 80 percent. As further proof of the perceived importance of R&D, note that whereas the overall U.S. defense budget increased by 38 percent (from $225.1 billion to $311.6 billion in real terms) from 1981 to 1987, military R&D spending increased by 100 percent (from $20.97 billion to $41.96 billion). Moreover, before World War II military R&D absorbed on average less than 1 percent of the military expenditure of major powers, but since then it has grown to 11–13 percent. The emphasis on military technology is bound to become more pronounced in the future as R&D becomes the main arena for interstate competition.

At least since Thucydides, students of international relations have sought rational explanations for the advent of war. Rationalist explanations assume purposive action; states are said to make reasoned decisions about the use of force. Although rationalist explanations have proven persuasive, durable, and offer the basis for cumulative theorizing, they also imply substantial limits on what we can know about war. I show that the most general rationalist explanation for war also dictates that the onset of war is theoretically indeterminate. We cannot predict in individual cases whether states will go to war, because war is typically the consequence of variables that are unobservable ex ante, both to us as researchers and to the participants. Thinking probabilistically continues to offer the opportunity to assess international conflict empirically. However, the realization that uncertainty is necessary theoretically to motivate war is much different from recognizing that the empirical world contains a stochastic element. Accepting uncertainty as a necessary condition of war implies that all other variables—however detailed the explanation—serve to eliminate gradations of irrelevant alternatives. We can progressively refine our ability to distinguish states that may use force from those that are likely to remain at peace, but anticipating wars from a pool of states that appear willing to fight will remain problematic. For example, we may achieve considerable success in anticipating crises, but our ability to predict which crises will become wars will probably prove little better than the naive predictions of random chance. The need for uncertainty to account for war means that the same conditions thought to account for war must also exist among states not destined to fight. Otherwise, states themselves will differentiate between opponents in a way that either removes the motives for war or restores uncertainty. It has long been accepted that social processes possess an element of uncertainty, but the centrality of uncertainty to rationalist explanations for war means that the advent of war is itself stochastic. War is literally in the “error term.”

Economic regionalism appears to be growing rapidly. Why this has occurred and what bearing it will have on the global economy are issues that have generated considerable interest and disagreement. Some observers fear that regional economic institutions—such as the European Union (EU), the North American Free Trade Agreement (NAFTA), Mercosur, and the organization of Asia-Pacific Economic Cooperation (APEC)—will erode the multilateral system that has guided economic relations since the end of World War II, promoting protectionism and conflict. Others argue that regional institutions will foster economic openness and bolster the multilateral system. This debate has stimulated a large and influential body of research by economists on regionalism's welfare implications.