The episode reflects poorly on his fitness to be president. Maybe he is bluffing—that is to say, maybe he wouldn’t, in fact, launch a possibly catastrophic war over mere words, preferring to ground such a momentous decision in the actual substance of American national security. If so, he spoke recklessly: America gains exactly nothing from a bluff of that sort, and risks exposure if Iran chooses to call him on his words. Trump foolishly put the U.S. in a reactive position.

Then again, maybe the president was serious. That would be much worse. That would mean he is willing to launch a potentially catastrophic war over something as inconsequential as a foreign leader saying, in effect: You’d better not go to war with us, you’ll regret it if you do. Starting wars over such meager threats would all but abolish peace.