This thesis, a genealogy of war as a problem of international politics, pursues two lines of inquiry. On the one hand, the thesis offers a genealogy of war’s becoming a problem to enable a critique of how war is presently constituted as problematic. The thesis analyzes the emergence of war as an object of empirical knowledge and practical action in four historical examples: a commission of inquiry into the Balkan Wars (1912-14), debates between international jurists in the aftermaths of the Franco-German War (1871-73) and the First World War (1920), and the post-Second World War International Military Tribunal for the Far East (1946-48). This genealogical analysis produces manifold empirical materials for getting to work on our contemporary ways of knowing about and acting upon war. Specifically, it points to these ways’ reliance on our being external to the problem of war. On the other hand, the thesis undertakes a conceptual and practical re-doing of genealogy. In the spirit of Foucault’s methodological conduct, the thesis foregoes genealogy as a pre-specified method. Instead, it constructs genealogy over the course of the research process: it conceptualizes genealogy as history/critique, problematization, and critical praxis, and it experiments with its own practices and ways of knowing. Understood and practiced in this way, genealogy pursues and actively tries to alter its entanglement with the problems that it studies. The thesis contributes to various International Relations (IR) literatures. To research on war as an object of knowledge and action, the thesis adds new historical insights and suggests a broadened critical outlook. To discussions about critical methods, the thesis contributes a historical, reflexive, and practical take on methods’ politicality and criticality. Finally, the thesis also makes a case for reconsidering IR’s assumptions about the problem of war and our capacity for knowing about and addressing it.