After his phone call with Mr. Erdogan, Mr. Trump ordered the American troops fighting alongside the Kurds in northeastern Syria to withdraw, lest they get in the way of the Turkish incursion. Defense Department officials pulled out about 50 American service members.

On Oct. 9, Turkish warplanes launched strikes on Kurdish positions in border towns in northeast Syria. On Oct. 10, the ground invasion began. On Oct. 11, Turkish forces shelled Kobani, a city close to an American Special Operations base. The American troops did not fire back, but they began withdrawing from the base a few days later, as Syrian and Russian troops entered to defend the town from advancing Turkish-backed forces.

On Oct. 13, Defense Secretary Mark T. Esper told CBS’s “Face the Nation” that Mr. Trump was ordering the remaining American forces out of northern Syria. “We have American forces likely caught between two opposing, advancing armies, and it’s a very untenable situation,” he said. The president, Mr. Esper added, had directed a withdrawal from northern Syria, which, he said, “is where most of our forces are.”

Mr. Trump affirmed his order on Twitter. “Others may want to come in and fight for one side or the other,” he tweeted. “Let them!”

By Oct. 14, the sentiment started to shift. Republican allies expressed outrage over Mr. Trump’s withdrawal order. Hundreds of Islamic State prisoners either escaped or were freed from detention in northern Syria, as Kurds turned their attention to fighting for their own survival. And Pentagon officials continued to argue to the president that the victory of the Islamic State was in danger of being reversed.

Mr. Trump, in a statement, said American troops “coming out of Syria will now redeploy and remain in the region to monitor the situation and prevent a repeat of 2014, when the neglected threat of ISIS raged across Syria and Iraq.” More significantly, Mr. Trump also announced that the American troops in the south, at Al-Tanf, would remain “to continue to disrupt remnants of ISIS.”

By Oct. 20, things were shifting again. Mr. Trump was talking about the need to protect the oil fields in eastern Syria. Pentagon officials began working on a plan to send additional American troops to guard oil fields.