Conflict prevention is a core item on the agenda of major international organizations and fora. In this article, I trace how war became a problem accessible to international governance. Adopting an object-centered approach to international relations (O-IR), I argue that war’s constitution as a problem of international governance unfolded in three interrelated processes. Firstly, pacifists and philanthropists designated war as a scientific object, thus giving rise to a “science of peace.” Secondly, scholars and pacifists compiled statistics on war, thus translating it across contexts and representing it as a global phenomenon. Statistics helped to make war accessible to advocacy and policymaking as an object of expertise. Thirdly, peace advocates problematized war as a governance object by representing it as a cost-benefit problem and a major cause in the reversal of economic development. By tracing the historical development of war as an epistemic object that can be investigated systematically, an object of expertise that can be measured and compared, and an object of governance that can be manipulated, this article bridges the strands of O-IR that have previously only focused on either knowledge and expertise or governance. Further, it adds to peace and conflict scholarship by providing an intellectual history of the prevention idea and its entanglement with modernism. Finally, it advances broader IR scholarship by offering an analysis of the role of scientific developments and nonstate activism in producing ideas and enabling policy agendas.