North Korea is still racing to achieve a comprehensive nuclear deterrent. And the closer it gets to its goal, the less time the United States and its allies have to try to stop it. Depending on factors such as the strength of U.S. intelligence, the progress of North Korea's missiles and nuclear programs and how much risk Washington and its allies are willing to tolerate, the United States may already have missed its opportunity for preventive military action. Official assessments indicate that, at most, Washington has 18 months before the window closes; after that, the United States and its allies probably will have no choice but to adopt a policy of deterrence toward North Korea. As the clock ticks down, we're constantly scanning the horizon for signs of an impending strike on North Korea, such as the evacuation of nonessential personnel from South Korea or a heightened alert level in the...