For much of the postwar period, the United Nations helped prevent crises from spiraling into wider war, acting as a firewall when bilateral diplomacy failed. As much of that history has faded from view, political leaders from around the world have come to think of the U.N. as an obsolete talking shop of empty words. This amnesia has narrowed the horizons of those shaping foreign policy, leaving them unable to envision a security framework that isn’t a zero-sum game of rival blocs. If we continue to let ourselves forget the lessons of the mid-20th century, when the U.N. was a successful bulwark against conflict escalation, we will find ourselves unable to imagine the kind of international cooperation needed to prevent future catastrophes.