Who benefits from Mr. Trump’s decision?

The immediate winners were Turkey and its Syrian Arab proxies, who had captured over 75 square miles of previously Kurdish-held territory by the end of the weekend.

The Islamic State might also profit from the instability, since Kurdish-led fighters no longer have the manpower to root out remaining militant cells or to guard roughly 11,000 captured ISIS fighters detained on Kurdish-held territory. The Kurds also operate more than a dozen camps for displaced families, in all holding tens of thousands of people, many of them the wives and children of Islamic State fighters.

The Syrian government is another beneficiary: On Sunday, the Kurdish authorities allowed Syrian troops to return to large parts of northern Syria in which they had no presence for more than half a decade.

Officially, the Syrian Army will just assist the Kurds in their defense of the area, with civilian life still managed by the Kurdish-led administration. But many fear that eventually the Assad regime, and its feared security forces, will take back control.

Russia and Iran, Mr. al-Assad’s main international protectors, are the other winners.

America’s withdrawal from northern Syria allows the two countries to expand their influence in the region. In particular, Russia has emerged as the main power broker in negotiations between the Kurds, Mr. al-Assad and the Turkish government.