Just last week, the presidents of Iran, Turkey and Russia joined hands at an international summit meeting in Ankara, Turkey, to celebrate their successes in Syria and plot their next moves. The United States, notably absent, had not even been invited.

By that time, Mr. Trump had suspended more than $200 million in funds for recovery efforts in Syria.

“I want to get out,” he said at the White House last week. “I want to bring our troops back home.”

Mr. Trump’s aides quickly talked him out of an immediate withdrawal. But Mr. Trump made clear that he wanted the troops out within a few months, senior administration officials said, a decision that would alter the landscape in ways that would echo far beyond Syria’s borders.

Foes of the United States have cheered the prospect of an American withdrawal. But America’s regional allies, including Israel, Saudi Arabia and its partners in Syria, dread it.

They argue that American forces are still needed to provide a check on Russia, which considers Syria its strategic foothold in the Middle East, and Iran, whose proxies are building a military infrastructure in Syria to counter Israel.

A withdrawal could also leave the door open for the return of the Islamic State in some parts of Syria, the very reason the United States gave for intervening in the country to begin with.