With the end of the cold war, the military posture of the North Atlantic Treaty Organization (NATO) has entered a period of profound change. Prior to the recent dramatic political events in Europe, however, NATO conventional force levels in the Central Region had been remarkably stable for some three decades. This article seeks to explain this record of stability in terms of three widely used theories of international relations. It argues that balance-of-power theory and public goods theory cannot alone provide a satisfactory account. Rather, these traditional approaches for understanding alliance behavior must be supplemented by regime theory, which emphasizes the constraining effects of enduring institutional factors even in the face of structural change. Specifically, it shows how an international regime has influenced the provision of conventional forces in the Central Region by alliance members. More generally, this analysis seeks to contribute to the literature on international regimes in three ways. First, it demonstrates that regimes do matter by providing an example of their importance for explaining state behavior and international outcomes. Second, it extends regime theory to relations among military allies. Third, it elaborates a comprehensive model for understanding why states actually comply with regime injunctions. The model stresses both the ways in which regimes effectively modify the international environment within which states operate, altering the costs and benefits associated with different courses of action, and the ways in which participating states may internalize regime norms and rules, thereby making compliance increasingly automatic.