This chapter examines the development of Reinhold Niebuhr’s thoughts on nationalism. Over the course of his lifetime, Niebuhr continually returned to the question of nationalism as a factor in international relations, revising his understanding in light of the particular circumstances confronting the United States and the global community. His early writings on German Americanism yielded to a more sceptical analysis of nationalism as a manifestation of collective egoism, but one which could nevertheless provide important resources to human communities. The threat of Fascist nationalism in the 1930s caused him to yet again revise his understanding of nationalism, as a revitalized form of democratic nationalism became necessary to confront it. The Cold War presented the context for Niebuhr’s mature reflection on the subject, advocating for a form of chastened nationalism, which was aware of both its responsibility to confront evil in the world, as well as its own tendencies towards self-delusion and the abuse of power.