Leaks like this happen, and it would have been nice—and a one-day story—if Mr. Trump had laughed it off, perhaps with a quip about how he’s closely following the continuing success of Brexit. But Mr. Trump never lets a slight go unanswered, and so he let loose a Twitter fusillade calling Mr. Darroch “a stupid guy” and “a pompous fool” whom he would no longer deal with. Oh, and he insulted the lame duck Mrs. May too.

In doing so he played into the hands of whoever in British politics (or whatever external enemy) leaked the memos to embarrass the government. Britain is a crucial U.S. ally and a major trading partner. Mr. Darroch’s cables were condescending, but Mr. Trump’s overblown response reminds the world how sensitive to criticism the President is and what a fickle friend he can be.

This won’t help him as he tries to build support for a united front against Iran, Russia or any other joint challenge. This behavior is one reason that many foreign leaders have concluded they will wait on hard decisions until 2020 to see if Mr. Trump is re-elected.