This article surveys the major historical developments in Western philosophical reflection on war. Section 2 outlines early development in Greek and Roman thought, up to and including Augustine. Section 3 details the systematization of Just War theory in Aquinas and his successors, especially Vitoria, Suárez, and Grotius. Section 4 examines the emergence of Perpetual Peace theory after Hobbes, focusing in particular on Rousseau and Kant. Finally, Section 5 outlines the central points of contention following the reemergence of Just War theory in the 1970s.