Following a discussion of what some scholars have seen as “divided loyalties” among immigrants, this article surveys the history of the ethnic influence on U.S. foreign policy from the 1790s to the present. Specific topics include Irish American nationalism and its relationship to republicanism and anti-imperialism in the nineteenth century, the diverse aims of ethnic activism in the era of World War I, the alleged role of ethnic activism in shaping the isolationism that partly characterized U.S. foreign policy between the wars, and the role of U.S.-based diasporas and ethnic lobbies in the era of the Cold War and after. Attention is given to Irish Americans, Jewish Americans, Cuban Americans, African Americans, Eastern Europeans, and others. The article closes with a discussion of the potential impact of the recent phenomenon of dual citizenship.