This article formulates a model of alliance persistence and evolution to account for two aspects of behaviour that are largely neglected by traditional alliance theory. It argues that states that encounter common threats to military and nonmilitary interests (like political and economic stability) are likely to institutionalize their military pacts. This process raises material and normative obstacles to exiting the alliance, even as the performance of the alliance falters or the strategic context undergoes fundamental changes. The article tests several of the model's hypotheses by engaging in a comparative analysis of two Cold War alliance systems: the North Atlantic Treaty Organization and the Southeast Asia Treaty Organization.