On the brink. That’s how we tend to think of humanity’s predicament during the most dangerous moments of the nuclear era. But as Thomas Schelling, the godfather of nuclear strategy, once pointed out, the phrase is misleading. The nuclear frontier is not “the sharp edge of a cliff where one can stand firmly, look down, and decide whether or not to plunge,” he wrote, but rather “a curved slope that one can stand on with some risk of slipping”—the slope getting steeper and riskier “as one moves toward the chasm.” Now the slope is getting steeper before our eyes.