Prominent theories of state formation hold that states formed because of warfare and competition on the one hand, or the diffusion of organizational templates and practices through learning and emulation on the other. We propose that the two strands of theory can be linked to more accurately account for mechanisms of state formation. War, we argue, is an important source of social diffusion. War establishes contacts between political elites across borders, generates migratory flows, and establishes new economic networks. We examine the validity of the theory through a comparative case study of Nordic political units from the dawn of the Viking Age to the end of the High Middle Ages (CE 800–1300), finding that raids, settlements, and conquests by Norwegian and Danish rulers in England, Europe's most advanced kingdom, set in motion state formation processes in Norway and Denmark. In these cases, the winners emulated the losers.