The decision makes as little sense strategically as it does morally. American allies from Berlin to Riyadh are alarmed. “Anyone who wants to assist Syria in protecting the Kurds is good with me, whether it is Russia, China, or Napoleon Bonaparte,” Mr. Trump wrote almost flippantly on Twitter on Monday . Yet at the same time that Mr. Trump has signaled that the Middle East should be someone else’s problem and has talked about bringing the troops home, he recently ordered another 3,000 to Saudi Arabia to deter Iran — which, like Russia and the Syrian government, has now only been emboldened by his flight from Syria.

History is littered with instances of one-time allies abandoned by Washington to their fate — the Bay of Pigs invasion; the fall of South Vietnam; numerous internal uprisings, like Hungary in 1956, that were fanned by the United States only to be smothered when aid, implicit or explicit, was withheld. The United States has abandoned the Kurds — a stateless people who live in parts of Turkey, Iraq, Syria, Armenia and Iran — on numerous occasions in just the past half century. The most infamous of these betrayals came when Saddam Hussein attacked them with poison gas in 1988, and the Reagan administration protected the Iraqi government from congressional sanctions.

Yet the decision by the Trump administration to quit Syria stands apart because the status quo was entirely sustainable. American forces were not taking high numbers of casualties. The region under control of the Kurds was largely quiet. Islamic State fighters were penned up. There wasn’t major international pressure for the United States to withdraw. If the Trump administration had wanted to acquiesce to Mr. Erdogan’s pleadings to let Turkey take stronger actions in service of its own national security, it could surely have managed such steps in a far more measured and coordinated manner.

Several Republicans, including Senator Lindsey Graham and Representative Liz Cheney, expressed outrage at the overnight betrayal of the Kurds. (Ms. Cheney went so far as to suggest that the Turkish military launched its invasion because Mr. Trump is weakened by an impeachment inquiry over his wielding of presidential power.) But they have only their party to blame for resisting any effort to hold the president accountable for his erratic navigation of American foreign policy or to temper policy decisions that have landed migrant children in cages at home and left longstanding alliances in tatters overseas.

If Mr. Trump hoped to improve relations with Turkey (where two Trump towers license his name, by the way) then he got that wrong, too. Anyone who could think half a step ahead would realize that any such warming would surely be chilled by the inevitable economic sanctions. In the wake of the invasion, the European Union opted to limit arms sales to Turkey, while sanctions under consideration on Capitol Hill could shut off flows of weapons, spare parts and ammunition from the United States.

At the moment, America’s priority must be to protect its soldiers in the field and secure its nuclear weapons. Turkey must understand that NATO will not come to its aid if its adventurism in Syria spins out of control and that the international community will reject any effort to dilute the Kurdish population by moving in other ethnic groups.

A few days ago there were valid options to answer the question of what the United States could do in response to the invasion. Harsh sanctions and other actions might have compelled Turkey to pull back, allowing for American troops to restore the status quo. Now the only alternative to Turkish control of the north is Mr. Assad’s control of the north. America’s alliance with Kurdish forces is probably dead, and it’s hard to see what role the United States can play in Syria or in the fight against the Islamic State. They say if you break it, you own it. But maybe all the United States has done is break it.