BEIRUT, Lebanon — Days after President Trump said he wanted to pull the United States out of Syria, Syrian forces hit a suburb of Damascus with bombs that rescue workers said unleashed toxic gas.
Within hours, images of dead families sprawled in their homes threatened to change Mr. Trump’s calculus on Syria, possibly drawing him deeper into an intractable Middle Eastern war that he hoped to leave.
“Many dead, including women and children, in mindless CHEMICAL attack in Syria,” Mr. Trump wrote on Twitter on Sunday. He blamed Iran and Russia — even singling out President Vladimir V. Putin of Russia by name — for their support of the Syrian government.
“Big price to pay,” he wrote, without providing details.
His homeland security adviser, Thomas P. Bossert, said the White House national security team had been discussing possible responses and would not rule out a missile strike.
The reported chemical attack on Douma, a suburb of the capital, Damascus, on Saturday seems to have squeezed Mr. Trump between conflicting impulses, and raised the political and military stakes as he charts the United States’ future in Syria.
On one hand, he has emphatically expressed his desire to bring American troops home as soon as possible in line with his “America First” approach. On the other, he has vowed to punish some bad actors, and withdrawing from Syria could open him up to criticism at home and abroad.
“The withdrawal of U.S. troops from Syria now would have major negative repercussions for the region and beyond,” said Murhaf Jouejati, a Syrian-American professor of international relations at the Emirates Diplomatic Academy in Abu Dhabi, United Arab Emirates.
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.
The bombing that Mr. Trump was responding to was part of a battle between rebels and the Syrian government, one that the United States had withdrawn from long ago, in a part of the country where the United States has neither a military presence nor any clear allies.
There is nothing to stop Mr. Trump from ordering a missile strike on western Syria, where the chemical attack was alleged to have taken place, and still pulling the approximately 2,000 American troops out of eastern Syria, where their main task was battling the militants of the Islamic State. But the seesaw of withdrawal and deeper engagement, without the articulation of a clear strategy for the region, is sure to confuse allies and enemies alike.
Some politicians, including Senator John McCain, Republican of Arizona, even argued that Mr. Trump’s talk of a rapid withdrawal had emboldened President Bashar al-Assad of Syria to use chemical weapons.
It was not clear on Sunday whether Mr. Trump’s tweets reflected serious planning for a military strike, or if the suspected chemical attack had changed his calculations about the necessity for a withdrawal of American ground forces. A White House official said he could provide no guidance beyond what the president had said on Twitter.
Mr. Trump’s furious response echoed his reaction to a similar attack that killed scores of people in northwestern Syria a year ago. Within three days of that attack, Mr. Trump sent a storm of cruise missiles raining down on the Syrian airfield where the attacks originated.
Graphic photos of the victims played a role in his decision then, as he expressed horror over images of “innocent children, innocent babies.” So did his repeated criticism of President Barack Obama for not intervening in similar circumstances.
But the strikes last April were an exception. Throughout the war, the United States has declined to intervene in Syria despite many attacks with much higher tolls.
“The Trump administration has ignored both conventional attacks on civilians and repeated use of chlorine gas against civilians which have claimed far more lives than this single horrific attack,” said Aaron David Miller, a former Middle East adviser in Republican and Democratic administrations and vice president of the Woodrow Wilson Center in Washington.
Seven years of fighting have shattered Syria into a series of overlapping conflicts among a kaleidoscope of combatants and international powers seeking to advance their interests.
Over the course of the war, Russia and Iran have committed deeply to Mr. Assad and have made major military, financial and political investments to ensure his survival. The United States initially called for Mr. Assad to leave power, and supplied cash and guns to the rebels seeking to oust him, but gave up hope long ago that they would prevail in toppling him.
The primary involvement of the United States now is in eastern Syria, where it formed an alliance with a Kurdish-led militia known as the Syrian Democratic Forces, or S.D.F., to fight the jihadists of the Islamic State.
The Islamic State has lost most of its territory, giving the American-backed forces control of a large area where they have set up local administrations and occasionally clashed with the Syrian government and its Russian and Iranian allies.
Those who support keeping American troops in Syria, including top American military officials, argue that they are needed to protect those gains.
Highlighting the internal debate in Washington, at virtually the same moment that Mr. Trump was saying it was time to leave, Gen. Joseph Votel, the head of the United States Central Command, was saying almost the opposite.
“The hard part, I think, is in front of us,” he said, suggesting that the United States should stabilize areas taken from the jihadists, return refugees to their homes and help with reconstruction. Currently, the United States military is clearing unexploded ordnance, mines and other booby traps from Raqqa just to make it minimally habitable. A withdrawal could jeopardize those endeavors.
The debate goes to the core of the American mission in Syria: whether United States troops are there solely to defeat the Islamic State militarily, or also to stabilize the areas the jihadists once ruled to keep them from coming back.
American military leaders have supported the latter option and consider the alliance with the S.D.F. an enduring strategic asset to accomplish that goal.
“We’ve always been there for them, and we’ll always be there,” Lt. Gen. Paul E. Funk II, the commander of the anti-Islamic State coalition, said in February during a meeting with a local military command in northern Syria.
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.”
Ben Hubbard reported from Beirut, Lebanon, and Julie Hirschfeld Davis from Washington. Contributing reporting were Rod Nordland from Manbij, Syria; Thomas Erdbrink from Tehran; Andrew Higgins from Moscow; Karam Shoumali from Berlin; and Hwaida Saad from Beirut.