When the UN Charter made it illegal for governments to go to war with each other, it created the first ever general legal ban on the use or threat of force by one state against another. This has been widely hailed as a dramatic contribution to world order – Michael Byers called it “one of the twentieth century’s greatest achievements.” My article on the ‘Permissive Power of the Ban on War’ opens up this conventional wisdom in two ways. First, I show that the UN Charter does not quite ban war - instead, it regulates the reasons that states may go to war. It is therefore as much a permissive rule as it is a constraining rule. Second, I suggest that much of the impact of international law in world politics comes through this permissive function, and that recognizing it is an important step toward appreciating the close connections between international legalization and global power politics.