In a major shift in United States military policy in Syria, the White House said on Sunday that President Trump had given his endorsement for a Turkish military operation that would sweep away American-backed Kurdish forces near the border in Syria.
Turkey considers the Kurdish fighters to be a terrorist insurgency, and has long sought to end American support for the group. But the Kurdish group, known as the Syrian Democratic Forces, or S.D.F., has been the United States’ most reliable partner in fighting the Islamic State in a strategic corner of northern Syria.
Now, Mr. Trump’s decision goes against the recommendations of top officials in the Pentagon and the State Department who have sought to keep a small troop presence in northeast Syria to continue counterinsurgency operations against the Islamic State, or ISIS, and to act as a critical counterweight to Iran and Russia.
Administration officials said that Mr. Trump spoke directly with President Recep Tayyip Erdogan of Turkey on the issue on Sunday. And the officials indicated that the 100 to 150 United States military personnel deployed to that area would be pulled back in advance of any Turkish operation but that they would not be completely withdrawn from Syria.
“Turkey will soon be moving forward with its long-planned operation into Northern Syria,” the White House said in a statement released just before 11 p.m. in Washington. “The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial ‘Caliphate,’ will no longer be in the immediate area.”
It was unclear how extensive the Turkish operation would be, or whether Turkish forces would clash with the American-backed Kurds, a development that could jeopardize many of the counterterrorism gains achieved by the American military in the fight against ISIS.
Last December, Mr. Trump called for a complete United States withdrawal from Syria, but ultimately reversed himself after a backlash from Pentagon, diplomatic and intelligence officials, as well as important allies in Europe and the Middle East.
Soner Cagaptay, the director of the Turkish Research Program at the Washington Institute for Near East Policy, and author of “Erdogan’s Empire: Turkey and the Politics of the Middle East,” said in a telephone interview that a Turkish incursion uncontested by the United States would allow Turkey to cut another swath into Kurdish-controlled territory in Syria. That would give Mr. Erdogan a ready place to send hundreds of thousands of Syrian refugees and prove yet again his influence with Mr. Trump on Syria policy.
“It’s a quite a significant development,” Mr. Cagaptay said.
Many Syria experts criticized the White House decision and cautioned that American abandonment of its Kurdish allies could widen the eight-year Syrian conflict and prompt the Kurds to ally with the Syrian government of President Bashar al-Assad to combat the much larger and more technologically advanced Turkish army.
“Allowing Turkey to move into northern Syria is one of the most destabilizing moves we can do in the Middle East,” Representative Ruben Gallego, an Arizona Democrat and former Marine who served in the Iraq war, said on Twitter on Sunday night. “The Kurds will never trust America again. They will look for new alliances or independence to protect themselves.”
Mr. Erdogan has demanded a “safe zone” for his nation to run 20 miles deep and 300 miles along the Turkish-Syrian border east of the Euphrates. That area, he has said, would be reserved for the involuntary return of at least a million Syrian refugees now inside Turkey. Mr. Erdogan has threatened to send a wave of Syrian migrants to Europe instead if the international community does not support the initiative to send them back to Syria.