This dissertation examines the contributions and significance of U.S. expatriates in Great Britain, West Germany, France, and Canada to the Vietnam antiwar movement. Utilizing archives of several expatriate antiwar groups, the personal papers of prominent expatriate activists, and the U.S. government, I argue dissent from this constituency was motivated by a desire to broaden U.S. civil society so that it included the perspectives, insights, and experiences of the highly mobile postwar population and accounted for the reality of its transatlantic empire. Overseas citizens often presented their dissent as patriotic, leaning on a range of national icons and traditions to situate themselves as part of the U.S. community, and, based on their experiences abroad, they claimed a specific expertise, unavailable to most other citizens on matters of foreign policy, international relations, and national security. As such, expats contested how U.S. policymakers used claims of national security and credibility to mobilize the transatlantic public for the war, and instead disseminated alternative interpretations as the basis of their dissent.