DOHUK, Iraq — Syrian government forces streamed into the country’s northeast on Monday, seizing towns where they had not stepped foot in years and filling a vacuum opened up by President Trump’s decision to abandon the United States’ Syrian Kurdish allies.
Less than a week after Turkey launched an incursion into northern Syria with Mr. Trump’s assent, President Bashar al-Assad of Syria, considered a war criminal by the United States, has benefited handsomely, striking a deal with the United States’ former allies to take the northern border and rapidly gaining territory without a fight.
In addition to Mr. al-Assad, Mr. Trump’s decision to pull United States forces out of the way has also quickly redounded to the gain of Russia and Iran, as well as the Islamic State, as the American retreat reconfigures battle lines and alliances in the eight-year war.
“For the Syrian regime and Russia, the Americans are leaving, so that is a big achievement,” said Hassan Hassan, a Syria analyst at the Center for Global Policy. “In just one day, gone. They don’t have to worry about what this presence means for the future.”
The greatest risk to American troops as they pull back comes from the Turkish-backed militia, the Free Syrian Army, which has spearheaded the Turkish offensive in many places, supported by Turkish Army artillery and mortar fire, and Turkish airstrikes.
American officials say these Turkish-backed militia are less disciplined than regular Turkish soldiers and, deliberately or inadvertently, have fired on retreating American troops.
In a sign of the concern over the safety of the remaining American troops in Syria, Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, spoke on Monday with his Russian counterpart about the deteriorating security in the country’s northeast.
Faced with a fast-unraveling situation, Mr. Trump’s policy toward the region continued to fishtail. Having essentially greenlighted the Turkish incursion a week ago, then threatening ruin to the Turkish economy, on Monday Mr. Trump announced that he would impose sanctions on Turkey, raising tariffs on steel and suspending negotiations on a $100 billion trade deal with Ankara.
Mr. Trump’s decision has turned a relatively stable corner of Syria into its most dynamic battleground. As Turkey and Syrian fighters it supports push in from the north to root out the Kurdish-led militia that was allied with the United States, Mr. al-Assad’s forces have moved in from the south, gobbling up territory.
On Monday, without a fight, government forces seized a number of towns that had recently been held by the United States’ allies, including Tel Tamer, home to an Assyrian Christian community; Tabqa, which has a large hydroelectric dam on the Euphrates River; and Ein Issa, where the United States kept a contingent of forces, until recently.
Fighting continued in towns near the Turkish border to the north, pitting a number of forces against each other and terrifying civilians.
Kurdish militiamen battled Turkish troops around Ras al Ain and Tal Abyad, Syrian border towns the Turks claim to have taken. And both Turkey and the Syrian government were sending troops toward Manbij, raising the specter of new fighting there.
President Recep Tayyip Erdogan of Turkey has said the incursion is necessary for his country’s security and that Turkey seeks to establish a 20-mile-deep “safe zone” for hundreds of miles inside Syria’s border.
The invasion has provoked widespread international condemnation and on Monday, the foreign ministers of all 28 European Union member states agreed to stop selling arms to Turkey, an unprecedented step toward a fellow NATO member.
But Mr. Erdogan appeared unfazed, vowing that Turkey would press on in a speech in Azerbaijan.
“We are determined to take our operation to the end,” he said. “We will finish what we started. A hoisted flag does not come down.”
Much of the territory contested in the current fighting was wrested from the jihadists of the Islamic State by an international coalition led by the United States in partnership with a Kurdish-led militia known as the Syrian Democratic Forces, or S.D.F. As the jihadists were rolled back, the S.D.F. seized its territory, which it sought to govern under protection from the United States.
But that partnership angered Turkey, which considers the Kurdish fighters terrorists for their links to a Kurdish guerrilla organization that has been fighting the Turkish state for decades.
It was Mr. Trump’s decision to pull American troops out of northern Syria that gave Turkey the opening to strike, setting off the current violence.
No longer protected by the United States, the Kurds struck a deal with the Syrian government, an American enemy, to bring its forces north to protect the area.
A Kurdish official, Aldar Xelil, said in a statement Monday that the agreement would put Syrian government forces on two strips along the border, but not in a section where Kurdish fighters are currently battling the Turks. The government forces would defend the border against the Turks, he said, while the Kurdish-led administration would continue to oversee governance and internal security in the region.
But much about the agreement remained unclear, and the Syrian state news media made no mention of it in its coverage of Syrian troops seizing towns and being welcomed by locals chanting in support of Mr. al-Assad.
About 1,000 United States troops serve on a number of bases throughout northeastern Syria, but President Trump’s orders will remove the troops over the next few weeks, sending them, at least initially, to Iraq. From there, they could be repositioned to other neighboring countries such as Jordan or Lebanon, or head back to the United States, military officials said.
For now, the Pentagon plans to leave 150 Special Operations forces at a base called al-Tanf, in southern Syria.
Trump administration officials had long argued that the troops were needed to check the influence of Iran, Russia and Mr. al-Assad; prevent the resurgence of the Islamic State; and give the United States leverage in eventual peace talks aimed at ending Syria’s war.
The administration has not explained how it plans to pursue these goals without troops or local allies in Syria.
Mr. Hassan, the Syria analyst, said it had become clear that Turkey and Mr. al-Assad had the most to gain from the American withdrawal and the reshuffling of Syria’s northeast.
Despite the international condemnation, Turkey had managed to quash the dream of Kurdish-led self-rule that had been growing for years in less than a week.
“This is the end or the beginning of the end of the Kurdish project in Syria,” Mr. Hassan said.
Mr. al-Assad and his Russian and Iranian backers had also won big because the United States had expended tremendous resources to defeat the Islamic State, and now Mr. al-Assad is poised to regain the territory.
“It is not just that you left, but that you did all this fighting on his behalf for the last five years,” Mr. Hassan said.
The biggest losers were the Kurds, who lost their foreign backers and saw their political dreams collapse, and the region’s civilians, who were now subject to yet another era of violence and uncertainty.
The new fighting in the north has displaced more than 160,000 people, according to the United Nations, limited access for aid organizations, and scattered families looking for safe places to wait out the violence.
Syrian refugees living in Turkey said they had lost contact with relatives in the border region as families had fled south into the desert hoping to avoid airstrikes and shelling by camping out in the open far from any cellphone coverage.
A Syrian house painter in the Turkish town of Suruc near the Syrian border said three of his wife’s cousins and another couple had disappeared and were thought to have been kidnapped on the road between Manbij and Raqqa.
“No one knows what is happening,” said the painter, Ali, who gave only his first name for fear of reprisals against relatives in Syria.
The loss of American support has terrified the region’s Kurds, many of whom distrust Mr. al-Assad but fear Turkey more.
Giving up the dream of self-rule would still be “a million times better than having our cities taken over by the terrorist mercenaries and criminal Turks,” said Arin Sheikhmous, a Kurdish activist in the border town of Kobani.
But as Mr. al-Assad’s forces advanced, others feared the horrors often associated with the Syrian state: conscription into the Syrian military or random arrests that have made untold numbers of people vanish into Syria’s prisons.
It remains unclear what will become of the more than 10,000 former Islamic State fighters held in Kurdish-run prisons, as well as the tens of thousands of women and children from the Islamic State now detained in squalid camps.
Some worried that the new deal between the Kurds and the government could see prisoners handed over to Mr. al-Assad.
Hamida Mustafa, a Syrian activist in southern Turkey, said he worried about his brother, who had been detained two years ago by the Kurdish-led militia.
He had heard on the first night of the Turkish incursion that the common criminals had been released while political prisoners had been moved to another prison in the city of Hasakah. He had not been able to locate his brother, but worried that he would end up being passed to the Syrian government and never seen again.
“We are fearful now,” he said. “They did that before.”