This article builds on theoretical work in the social movements literature that uses “master frames” (Snow and Benford 1992) to account for the cyclical clustering of social movement activity within certain historical periods. I identify “master frame alignment” as the dynamic process by which social movement actors rhetorically transform the master frames within a cycle of protest to make them resonate more clearly with a movement's unique social and historical situation. Just as frame alignment processes serve to link a movement organization's activities, goals, and ideology with those of a potential group of adherents, master frame alignment processes link the activities, goals, and ideology of a movement organization with those espoused within the broader symbolic atmosphere of the social movement. I present historical data from Irish newspapers and political documents to show how the Irish Sinn Féin movement, seeking self-determination during the early twentieth century, rhetorically reconstructed the master frames generated by the League of Nations in order to better exploit this particular window of political opportunity.