In this paper, we explain why the U.S. government chose multilateral security arrangements in Europe and bilateral ones in Asia in the 1940s and 1950s. After reviewing the inadequacies of a number of universal and indeterminate explanations, we put forward three explanations—great power status, efficient responses to threats, and regional identity—which rely on the combination of material and social forces for their explanatory power. Starting with common rationalist explanations that focus on material capabilities and institutional efficiency to explain the forms of international cooperation, we add to them the important effect that America's collective identity had on the formulation of its foreign policy goals. U.S. policymakers believed that the United States was a natural part of the North Atlantic community but that Southeast Asia was part of an alien political community. This difference helped drive the U.S. government to adopt divergent policies in two regions that, far from being natural, were constructed politically only in the 1940s. We conclude by pointing to the advantage of eclectic combinations of rationalist and constructivist insights, with an extension to the politics of regional collective identity in the 1990s.