The withdrawal will abandon Syria’s Kurds, who have borne the brunt of the battle against Islamic State, to the tender mercies of the Turks, who view them as terrorists. It will expose Syrians in Idlib and elsewhere to renewed attack from Bashar Assad’s forces. It will leave the Israelis face-to-face with the threatening Iranian presence in Syria. It betrays everyone who relied on the word of the U.S. And it hands a major victory to Vladimir Putin, who sees Syria as the linchpin of his effort to restore Russian influence in the Middle East.

It is possible, I suppose, that Mr. Trump does not understand the difference between the government of the U.S. and a family business, and so does not understand why his mercurial behavior inflicts so much damage. When you are in charge of the executive branch, you cannot do everything on your own. You need an orderly policy process and senior officials who can implement the results of this process. These officials cannot be effective unless they can credibly claim to be acting on your behalf and with your support. When your national security adviser announces that a policy enjoys your imprimatur, the world will take him at his word—unless you contradict him and pull back the policy. Silence implies consent.