The United States abandoned half of Europe to the Soviets, which included abandoning the Free Poles who fought by our side in World War II. We let South Vietnam go down the drain in a fit of Watergate pique. George H.W. Bush called on the Iraqi people to remove Saddam Hussein and then allowed them to be massacred without U.S. assistance. The U.S. set matters right with the 2007 surge in Iraq whereupon President Obama withdrew the troops and let the place fall apart again. Mr. Obama also assured Syrian oppositionists that the U.S. would respond militarily if Bashar Assad used chemical weapons and then didn’t.

It would be nice if the U.S. could provide steady, wise and endlessly resourceful leadership. It would be nice if each of us, in our own lives, could be everywhere and do everything. That’s not life.

Lots of people think they know everything they need to know about Donald Trump’s withdrawal of U.S. forces from a corner of Syria by who made the decision. It was impulsive, shameful, self-serving and wrong, because it was made by Mr. Trump. But even a man who pays little attention to his briefings would have heard by now that the U.S. position presented an untenable dilemma.

Turkey is an Article 5 NATO ally. We have obliged Ankara for two decades by designating its Kurdish separatists as terrorists. The U.S. has never advocated breaking up Syria (or Iraq, Iran or Turkey) to allow a Kurdish state. U.S. support for an autonomous Kurdish enclave in Iraq was dependent on the Kurds’ recognizing Baghdad’s sovereignty and not using Iraqi Kurdistan as a base to subvert neighboring states.

We talk loosely about U.S. troops being a tripwire, but we don’t actually leave soldiers vulnerable to being run over by a hostile force. If Turkey threatens to invade northern Syria, the U.S either has to remove its troops or back them up with enough force to protect them from the Turks.