This chapter explores the justness, legitimacy, and legality of war. While 1945 was a key turning point in the codification of jus ad bellum (i.e. international law on the use of force), that framework did not emerge in a vacuum. Rather, it was the product of historical political contingencies that meant that codification of the laws of war was contemporaneous, both geographically and temporally, with the solidification of the norms of sovereign nation-statehood and territorial integrity. The chapter focuses on the UN Charter regime and how it has shaped the politics of war since 1945. Importantly, the Charter establishes a general prohibition on the use of force in international relations. It also grants two exceptions to the prohibition: actions undertaken with Security Council authorization and actions taken in self-defence. Today, many of the most serious challenges to the Charter regime concern the definition and outer limits of the concept of self-defence. Another set of challenges to the Charter regime concerns the contested concept of ‘humanitarian intervention’. The chapter then looks at the development of the ‘Responsibility to Protect’ doctrine.