President Trump’s defeat of Islamic State as a territorial power was a major foreign-policy success, yet he may now undo it with a retreat from Syria that will also signal to U.S. allies that the White House can’t be trusted.

That’s the risk of Mr. Trump’s abrupt decision late Sunday to abandon northern Syria to Turkey. Washington and Ankara had been negotiating to create a buffer zone to avoid a conflict there, but on Sunday the White House announced that American forces will cede the area to Turkish troops. Turkish President Recep Tayyip Erdogan is now free to wage war on Syria’s Kurds, who were America’s most important allies against ISIS.

Mr. Erdogan says the U.S.-armed Kurdish fighters in Syria, known as the YPG, have ties to the Kurdistan Workers’ Party, a domestic Kurdish insurgency within Turkey. On that exaggerated claim he justifies an exercise that could amount to ethnic cleansing. Mr. Erdogan last month proposed a “safe zone” extending some 20 miles into Syria from the Turkish border, where he would resettle millions of Syrian refugees. This could require the forcible resettlement, or worse, of Kurds already living in the area.

Mr. Erdogan seems to have believed that the U.S. would help in this exercise. But after a phone call with the Turkish strongman, Mr. Trump made clear that Turkey is on its own. That also means so are the Kurds, and the U.S. withdrew its troops from two border posts. A Kurdish spokesman tweeted, “We are not expecting the US to protect NE #Syria. But people here are owed an explanation.”

This looks like a betrayal of the YPG, which lost 11,000 soldiers fighting against ISIS. America armed the Kurds in that fight, and they trusted the U.S. when they were asked to dismantle defensive positions near the Turkish border as part of the buffer-zone negotiations with Ankara. The Kurds are less likely to aid an insurgency in Turkey if they’re allowed to govern themselves in a safe area in Syria policed by the U.S. and Turkey.