Abandoning the force, which is led by Kurds, would also leave it at the mercy of other powers, particularly Turkey, which considers it terrorist and a threat to Turkish sovereignty.
Turkey invaded the Kurdish enclave of Afrin in northern Syria, seizing it last month, and has threatened to move east to areas where American troops now operate, leading to the possibility of NATO allies fighting each other. The Turkish invasion has also siphoned Kurdish fighters away from the fight against the Islamic State in the south, slowing it down, American officials say.
One of the greatest beneficiaries of an American withdrawal would be Iran, a country that Mr. Trump has blamed for many of the troubles in the Middle East and has vowed to confront.
“It’s simple: If American forces leave Syria, there will be more room for Hezbollah and Iran to maneuver,” said Hamidreza Taraghi, a hard-line analyst in Iran.
Russia also cheered the prospect of an American withdrawal.
“The less American interference, the fewer American soldiers, the better for everyone,” said Andrei A. Klimov, deputy head of the foreign affairs committee of Russia’s upper house of Parliament. An American retreat would cement Russia’s status as a power to be reckoned with in the Middle East and would further burnish Mr. Putin’s reputation as a master tactician on the world stage.
Yet an American pullout could create headaches for Mr. Putin, who has repeatedly declared “mission accomplished” in Syria but has still not delivered on repeated pledges to draw down Russia’s military forces. An American withdrawal could also leave Russia stuck with the reconstruction bill for a country where many cities and most of the infrastructure have been destroyed.
Mr. Putin has called on “massive capital investments” from wealthy countries to help rebuild Syria, saying they need to become “more actively involved in deed and not only in word.”
But Western nations are unlikely to support the project as long as Mr. Assad, whom many consider a war criminal, remains in power.
Hanging over the decision to stay or leave is a the bitter American experience in Iraq.
In 2011, after years of heavy military engagement there, the United States declared victory over the Iraqi insurgency — the predecessor to the Islamic State — and left. Three years later, the jihadists returned, stronger than before, and took over a third of Iraq and a large part of Syria.
Some of the United States’ regional allies, and many of its own officials, believe the United States should remain in Syria to prevent that history from repeating itself. Merely talking about leaving could plant the seeds of that resurgence.
“Any decision to withdraw the American efforts from Syria now is not realistic or of a timely manner as the threat by terrorists groups is still there,” said Shahoz Hasan, a leader of the primary Kurdish political party in Syria. “Jihadists are looking forward to such a decision to be able to breathe again and reorganize their groups.”