On Sunday, a senior administration official would not say what that ultimately meant for the timetable for troops in Syria, but said the president had reiterated to Mr. Erdogan that the United States would remain there long enough to ensure an orderly handover and “help out logistically” to eradicate any territory still held by the Islamic State.
The official spoke amid reports that Turkey was moving troops near a town in northern Syria held by Kurdish allies of the United States, even though Turkey had said it would put off a promised offensive after Mr. Trump’s hasty decision to leave Syria.
Mr. Mattis resigned on Thursday in large part over that pullout order. The defense secretary was also upset about Mr. Trump’s decision to bring home half of the 14,000 American troops stationed in Afghanistan and his order to deploy American troops to the border with Mexico.
The president has grown increasingly angry as commentators have described Mr. Mattis in near heroic terms for standing up to Mr. Trump and making his resignation count as no one else in the president’s circle has done, the aide said. On Saturday, Mr. Trump took a jab at Mr. Mattis on Twitter, saying that “when President Obama ingloriously fired Jim Mattis, I gave him a second chance. Some thought I shouldn’t, I thought I should.”
By Sunday morning, Mike Pompeo, the secretary of state, had informed Mr. Mattis that he would have just over another week in the job. Mr. Pompeo and John R. Bolton, the president’s third national security adviser since taking office, are left to direct policy while the president considers an official replacement for Mr. Mattis. In a call with reporters, a White House official framed Mr. Shanahan’s tenure as one that could keep daily operations stable in the interim.
Brett H. McGurk, the special presidential envoy to the coalition fighting the Islamic State, is also stepping down over Mr. Trump’s decision to pull troops from Syria, telling colleagues this weekend that he could not in good conscience carry out Mr. Trump’s new policy.
Mr. McGurk, a seasoned diplomat who was considered by many to be the glue holding together the sprawling international coalition fighting the terrorist group, was supposed to retire in February. But according to an email he sent to his staff, he decided to move his departure forward to Dec. 31 after Mr. Trump did not heed his own commanders and blindsided America’s allies in the region by abruptly ordering the pullout.