For decades, American allies in Asia and Europe have relied on U.S. extended nuclear deterrence for their safety. They forswore acquiring their own nuclear weapons, agreeing instead to live under the protection of the United States’ nuclear umbrella. This arrangement worked during the Cold War because the stakes of that competition were so high that the United States could credibly—if just barely—say that it would wage nuclear war, even at catastrophic risk to itself, to prevent the conquest of