In announcing the planned American withdrawal from Syria, Mr. Trump declared, “We have won against ISIS.”He made the decision after speaking with President Recep Tayyip Erdogan of Turkey, who assured Mr. Trump that his country could mop up what was left of the Islamic State. But many American officials warned against abandoning the United States’ Kurdish allies; Mr. Erdogan wants to defeat them, as well, seeing them as tied to a Kurdish insurgency within Turkey.Mr. Trump and Mr. Erdogan spoke by phone on Sunday, and the Turkish leader said his forces were ready to take over security in Manbij without delay, Anadolu, the semiofficial Turkish news agency, reported.A White House statement and the Anadolu report stressed that the two presidents had agreed to work together against terrorism. Anadolu also said that Mr. Erdogan had told Mr. Trump that he would not allow Kurdish forces in northeastern Syria to be a destabilizing force.The White House put a more conciliatory gloss on that issue, saying, “the two leaders agreed to pursue a negotiated solution for northeast Syria that achieves our respective security concerns.”Mr. Erdogan also expressed condolences for the American deaths last week, Anadolu reported, calling the attack a provocation to influence the United States withdrawal.On Monday, a State Department spokesman, Robert Palladino, said that Secretary of State Mike Pompeo had spoken with his Turkish counterpart, Mevlut Cavusoglu, and discussed “the deliberate and coordinated withdrawal of U.S. forces from Syria.”Mr. Pompeo also spoke about the United States’ commitment to addressing Turkey’s security concerns, Mr. Palladino said, and “the protection of forces” that have worked with the United States-led coalition against ISIS.