Three days before Christmas, President-elect Trump tweeted (yes, tweeted) that the U.S. “must greatly strengthen and enhance its nuclear capability” until the world “comes to its senses regarding nukes.” The world, for its part, blinked in astonishment, wondering once again what Mr. Trump might mean, and why such a momentous announcement appeared via social media. Prior presidents generally undertook any shift in nuclear policy with care, and with the advice of experts in arms control and proliferation who have made keeping us safe their life’s mission. After all, when a single person has the power to rain down nuclear fire...