This article examines cross-national variation in the diffusion and adoption of military technologies and ideas. The history of warfare has been marked by periods of innovation in which the institutions and practices of war-making adapted in response to technological opportunities, and social and political developments. As information about new practices spreads, through the demonstration effects of innovating states or transnational social networks, military innovations have diffused throughout the international system. Diffusion can restructure power relations as states leverage new capabilities to increase their military power and enhance their international influence.