The long peace of the past eight decades has rested on two revolutionary convictions: that wars of aggression are intolerable and that empires must end. The first principle emerged from the carnage of two world wars, which together killed a hundred million people. The second came from centuries of colonial subjugation and the fight across Asia, Africa, and Latin America for self-determination. The United Nations Charter, signed in San Francisco in June 1945, gave both convictions political form. Since then, the world has avoided a cataclysmic great-power war. Even more remarkably, global European empires were dismantled and replaced by a new system of nearly 200 sovereign states. Both achievements combined to make possible extraordinary advances in human well-being. To be sure, the world has witnessed many conflicts since the end of World War II, including savage wars of decolonization, and soaring economic growth has come alongside deep inequalities and environmental destruction.…