The Winter Olympics’ closing ceremonies also concluded North Korea’s propaganda effort to divert attention from its nuclear-weapons and ballistic-missile programs. And although President Trump announced more economic sanctions against Pyongyang last week, he also bluntly presaged “Phase Two” of U.S. action against the Kim regime, which “may be a very rough thing.”

CIA Director Mike Pompeo said in January that Pyongyang was within “a handful of months” of being able to deliver nuclear warheads to the U.S. How long must America...