For decades after the Cold War ended, the threat of nuclear war seemed to fade into the global background. Climate change took center stage as the existential crisis of our time, and it seemed for a few brief years that treaties and diplomacy, however flawed, had led nuclear powers to set aside the possibility of using nuclear weapons again. (To date, it is only the U.S. that has detonated nuclear weapons—both in Japan—and it continues to be the country with the largest nuclear arsenal by far.)