There hasn’t been a major nuclear incident, outside of accidental meltdowns, since World War II. This is no small miracle, and there’s reason to wonder when this string of good luck—because it has included many near-disasters—will end.

The U.S. recently formally announced that it will withdraw from the Intermediate-range Nuclear Forces (INF) Treaty with Russia if Russia doesn’t come into compliance, and at some point we could still face a showdown over nuclear disarmament on the Korean Peninsula. Further adding to the risks...