My dissertation presents the first systematic study of “internationalized territories,” or spaces in which multiple external actors cooperate to exercise sovereignty. This dissertation identifies, defines, and theorizes internationalization as a distinct form of political rule in which territorial exclusivity is abandoned to preserve geopolitical stability. It does so by freezing conflicts that cannot be resolved through war, partition, or the creation of a buffer state. The dissertation develops an original framework to explain when and why internationalization arises, how it functions, and how it ends. The first chapter identifies three historical periods in which internationalization has occurred, each characterized by historically-specific configurations of actors, interests, and constraints. I argue that each of these three periods are responses to different “problems” associated with the global transformation from a world of empires to a world of nation-states. This variation in the problems being addressed by territorial cooperation leads to variation in the solutions, or the institutions of internationalization, across these periods. The second chapter examines why internationalization is pursued over alternatives that preserve territorial sovereignty, such as partition. This analysis builds upon theories of buffer states in international relations theory, characterizing internationalization as occurring when partition or independence are not feasible. Finally, the third chapter identifies three ways in which internationalized territories end and discusses their consequences, arguing that while internationalization may be a useful short-run mechanism for postponing conflict, it is not sustainable as a permanent solution. By recovering internationalization as a distinct political form, the dissertation contributes to debates on sovereignty, territoriality, and the evolution of international order. It challenges linear models of post-imperial transition and highlights the role of improvised, cooperative authority in managing territorial disputes. Ultimately, it demonstrates that internationalization persists not because it offers a clear path forward, but because it offers a last- resort mechanism to delay conflict when no other alternative is viable.