The historical rise of European foreign, security, and defense policy marks an important development in European politics and world politics more broadly. Long thought unlikely to amount to much, European integration in the domains of traditional “high politics” has consolidated bit by bit since the mid-1990s, under the auspices of a common foreign and security policy (CFSP) and a pan-European security and defense policy (ESDP). Accordingly, European affairs in these areas have attracted increased scholarly interest. In a variety of books as well as journal articles, scholars from diverse theoretical and intellectual backgrounds have argued that European integration in these policy areas has gained considerable substance—while offering very different causal accounts for why this has occurred. These different theoretical and empirical investigations together produce a new field of study with its own research questions, vocabulary, and search for causal explanations. IR theory is now engaging fully with European integration studies and vice versa. Paradoxically, this takes place in precisely those policy areas in which European integration had long been the weakest and least developed. This article explores and evaluates this new literature that analyzes why, compared with even the very recent past, a European foreign and security policy has emerged and apparently solidified.