A basic debate in world politics involves the impact of intergovernmental organizations (IGOs) on international conflict. Liberals, functionalists, and others see IGOs as capable of transforming global anarchy, while realists emphasize the essential irrelevance of IGOs in managing such fundamental processes as war and peace. Recent quantitative studies also yield disparate conclusions depending on particular econometric assumptions, implying variously that IGOs foster pacific relations among states, have no impact on dispute behavior, or even increase dispute propensity. At least part of the problem is a lack of theoretical and empirical specificity. The authors apply bargaining theory to develop a “middle path” between the realist and liberal perspectives. Only some IGOs, those with security mandates and the most sophisticated institutional structures, are likely to influence dispute behavior. The authors combine the theory with two improvements in research design. First, IGOs vary in capability, mandate, and cohesion. The authors construct a dataset of IGO institutional heterogeneity and member cohesiveness. Second, states join IGOs for reasons that are not unrelated to why states fight. The authors control for the level of international involvement among countries and find support for their arguments in initial tests.