This article appeals to classical realism for new insights into the role emotions play in shifting the terrain of political allegiance in global politics. Although undetected in readings emphasizing rational statecraft, realists such as Hans Morgenthau and Reinhold Niebuhr were centrally concerned with human emotions and their political impact. While following the intellectual currents of their time in regarding emotions as fixed impulses, these realists’ deep appreciation for the contingencies of history also led them to cast emotions as socially conditioned mechanisms of adaptation. By revisiting the texts of classical realism, this paper develops a fresh account of how emotion responds to and engenders change in the social world – in particular, change in the location of political allegiances. I then show how Morgenthau and Niebuhr applied these ideas not only to the nation-state but also to the most vexing transnational phenomena of their time – communism and liberal internationalism. In conclusion, the paper speculates that these reflections on dynamic allegiances at the transnational level offer realists and other international relation theorists insight into the emotional appeal, adaptability, and organizational complexity of contemporary non-state movements and actors.