As recently as Sunday, President Trump’s national security adviser, John R. Bolton, had said that the pullout was conditional on circumstances that could leave American forces there for months or even years. That followed by less than a month President Trump’s announcement that he intended to pull out the roughly 2,000 American troops within 30 days.

The surprise announcement came in a statement from Col. Sean Ryan, the spokesman for the United States-led coalition against the Islamic State. Colonel Ryan said the coalition had “begun the process of our deliberate withdrawal from Syria,” adding that he would provide no further information about “specific timelines, locations or troop movements.”

But the announcement, coming days after Mr. Bolton’s remarks, added to a climate of chaos surrounding Washington’s policy on Syria at a time when Turkey has threatened to invade the country.