This article seeks to overturn the conventional wisdom that World War II forced a decisive, bipartisan break in American grand strategy. As they had after World War I, American political elites debated the relative merits of unilateralism and multilateralism. Assessments of the relative costs and benefits of a cooperative and multilateral solution to American security depended on judgments about the likelihood of opportunism by America’s partners. Democrats were more trusting than Republicans, expecting cooperation where the latter anticipated defection. This led to different preferences for the creation and design of the United Nations and the North Atlantic Treaty. Drawing on theories of “social orientation” and political ideology, I explain why the left is more trusting than the right. Rationalist accounts of the creation and design of the UN and NATO overstate the case for ideological convergence and therefore the importance of structure because they largely ignore behind-the-scene bipartisan consultations that allowed for a compromise prior to the votes on the respective treaties. My social psychological theory of international cooperation demonstrates that multilateralism is a dispositional trait, not a simple functional response to some objective security situation.