The subject of this thesis is European armaments cooperation between 1992 and 2005. During this period, European governments moved the institutional forum of cooperation from a transatlantic to a European context, extended joint activities to new areas, and established several organizations. Moreover, some of the largest defense companies merged cross-border, and the European Commission started playing an increasing role. The scholarly literature explains these new patterns of cooperation as a result of economic pressures but fails to account for the uneven effects of industrial consolidation, the irregular dynamic of cooperation, the partly overlapping responsibilities of the organizations, and leaves little room for political agency. The thesis conceptualizes armaments cooperation as a setting of a particular problématique and a related regime. It argues that a specific regime configuration corresponds to a certain understanding of the problem at hand, the political project at stake, and a perspective that aligns the joint activities. In 1992 and 1998, different groups of governments conceived of different problems that ought to be solved through cooperation. Hence, the regime was divided into separate arenas inside and outside the EU, in which joint activities were pursued with different purposes, leading to different degrees of integration, and the establishment of partly competing organizations. In 2005, all European governments accepted a link of the problems solved through armaments cooperation to the political project of establishing the EU as an international actor. They agreed that the purpose of cooperation was to provide the necessary military and technological capabilities. Consequently, cooperation was pursued in an increasingly integrated EU arena, inspired by a capabilities perspective, and supported by international organizations like the European Defence Agency and the Commission. Political leadership, timing, and concept crafting are identified as three aspects that are critical for shifts of the problématique. The thesis develops a theory neither of armaments cooperation nor of these cognitive shifts but stresses instead that political phenomena should be made intelligible, with reference to the context and time bound understandings of political actors.