We are entering a new and dangerous period in which nuclear competition or even use of nuclear weapons could again become the greatest threat to global stability. Less certain is whether today’s leaders are up to meeting this emerging challenge. Until just a few years ago, it looked as if the problem posed by nuclear weapons had been successfully managed, if not solved. American and Russian nuclear stockpiles had been reduced substantially from their Cold War highs, and arms-control agreements were in place that limited both intermediate- and long-range systems. But all of this now could come undone. Progress over…