WASHINGTON — American commandos were working alongside Kurdish forces at an outpost in eastern Syria last year when they were attacked by columns of Syrian government tanks and hundreds of troops, including Russian mercenaries. In the next hours, the Americans threw the Pentagon’s arsenal at them, including B-52 strategic bombers. The attack was stopped.
That operation, in the middle of the American-led campaign against the Islamic State in Syria, showed the extent to which the United States military was willing to protect the Syrian Kurds, its main ally on the ground.
But now, with the White House revoking protection for these Kurdish fighters, some of the Special Forces officers who battled alongside the Kurds say they feel deep remorse at orders to abandon their allies.
“They trusted us and we broke that trust,” one Army officer who has worked alongside the Kurds in northern Syria said last week in a telephone interview. “It’s a stain on the American conscience.”
“I’m ashamed,” said another officer who had also served in northern Syria. Both officers spoke on the condition of anonymity to avoid reprisals from their chains of command.
And the response from the Kurds themselves was just as stark. “The worst thing in military logic and comrades in the trench is betrayal,” said Shervan Darwish, an official allied with the Kurdish-led Syrian Democratic Forces.
The next flurry of orders from Washington, as some troops had feared, will pull American troops out of northern Syria altogether. Defense Secretary Mark T. Esper said on Sunday that President Trump had ordered the roughly 1,000 American troops in the country’s northeast to conduct a “deliberate withdrawal” out of the country in the coming days and weeks.
The defense secretary’s statement came after comments on Friday pushing back on complaints that the United States was betraying allies in Syria — “We have not abandoned the Kurds” — even as he acknowledged that his Turkish counterpart had ignored his plea to stop the offensive.
Army Special Forces soldiers — mostly members of the Third Special Forces Group — moved last week to consolidate their positions in the confines of their outposts miles away from the Syrian border, a quiet withdrawal that all but confirmed the United States’ capitulation to the Turkish military’s offensive to clear Kurdish-held areas of northern Syria.
But as the Americans pulled back, the Kurds moved north to try to reinforce their comrades fighting the offensive. The American soldiers could only watch from their sandbag-lined walls. Orders from Washington were simple: Hands off. Let the Kurds fight for themselves.
The orders contradicted the American military’s strategy in Syria over the past four years, especially when it came to the Kurdish fighters, known as the Y.P.G., who were integral to routing the Islamic State from northeastern Syria. The Kurds had fought in Manbij, Raqqa and deep into the Euphrates River Valley, hunting the last Islamic State fighters in the group’s now-defunct physical caliphate. But the Syrian Democratic Forces, or S.D.F., as the Kurdish and their allied Arab fighters on the ground are called, are being left behind.
American Special Forces and other troops had built close ties with their Kurdish allies, living on the same dusty compounds, sharing meals and common dangers. They fought side by side, and helped evacuate Kurdish dead and wounded from the battlefield.
“When they mourn, we mourn with them,” Gen. Joseph L. Votel, a former head of the military’s Central Command, said on Thursday at the Middle East Institute.
The Kurdish forces and American military have survived previous strains, including Mr. Trump’s sudden decision in December to withdraw all American troops from northern Syria, a decision that was later walked back somewhat.
This time may be different, and irreversible. “It would seem at this particular point, we’ve made it very, very hard for them to have a partnership relationship with us because of this recent policy decision,” General Votel said.
As part of security measures the United States brokered to tamp down tensions with Turkish troops, Kurdish forces agreed to pull back from the border, destroy fortifications and return some heavy weapons — steps meant to show that they posed no threat to Turkish territory, but that later made them more vulnerable when Turkey launched its offensive.
Special Forces officers described another recent operation with Kurds that underscored the tenacity of the group. The Americans and the Kurdish troops were searching for a low-level Islamic State leader in northern Syria. It was a difficult mission and unlikely they would find the commander.
From his operations center, one American officer watched the Kurds work alongside the Americans on the ground in an almost indistinguishable symmetry. They captured the Islamic State fighter.
“The S.D.F.’s elite counterterrorism units are hardened veterans of the war against ISIS whom the U.S. has seen in action and trust completely,” said Nicholas A. Heras, a fellow at the Center for a New American Security, who visited the S.D.F. in July to advise them on the Islamic State, or ISIS.
During the battle against ISIS, coordination between the United States military and the Syrian Democratic Forces has extended from the highest levels to rank-and-file fighters, according to multiple interviews with S.D.F. fighters and commanders in Syria over the course of the campaign.
S.D.F. commanders worked side by side with American military officers in a joint command center in a defunct cement factory near the northern Syrian town of Kobani, where they discussed strategy and planned future operations.
The battle of Kobani that began in 2014 gave birth to the United States’ ties to the Kurds in northeastern Syria. ISIS fighters, armed with heavy American-made artillery captured from retreating Iraqi army units, surrounded Kobani, a Kurdish city, and entered parts of it.
Despite the Obama administration’s initial reluctance to offer help, the United States carried out airstrikes against advancing ISIS militants, and its military aircraft dropped ammunition, small arms and medical supplies to replenish the Kurdish combatants.
That aid helped turn the tide, the Kurds defeated ISIS, and American commanders realized they had discovered a valuable ally in the fight against the terrorist group.
Thousands of S.D.F. fighters received training from the United States in battlefield tactics, reconnaissance and first aid. Reconnaissance teams learned to identify Islamic State locations and transmit them to the command center for the American-led military coalition to plan airstrikes.
Visitors to front-line S.D.F. positions often saw Syrian officers with iPads and laptops they used to communicate information to their American colleagues.
“For the last two years, the coordination was pretty deep,” said Mutlu Civiroglu, a Washington-based Kurdish affairs analyst who has spent time in northeastern Syria. “The mutual trust was very high, the mutual confidence, because this collaboration brought enormous results.”
“They completed each other,” he said of the S.D.F. and United States-led coalition. “The coalition didn’t have boots on the ground and fighters didn’t have air support, so they needed each other.”
That coordination was critical in many of the big battles against the Islamic State.
To open the battle in one town, S.D.F. fighters were deposited by coalition aircraft behind the Islamic State’s lines. At the start of another battle, United States Special Operations forces helped the S.D.F. plot and execute an attack across the Euphrates River.
Even after the Islamic State had lost most of its territory, the United States trained counterterrorism units to do tactical raids on ISIS hide-outs and provided them with intelligence needed to plan them.
Even in territory far from the front lines with the Islamic State, S.D.F. vehicles often drove before and after American convoys through Syrian towns and S.D.F. fighters provided perimeter security at facilities where United States personnel were based.
The torturous part of America’s on-again, off-again alliance with the Kurds — one in which the United States has routinely armed the Kurds to fight various regimes it viewed as adversaries — emerged in 1974, as the Kurds were rebelling against Iraq. Iran and the United States were allies, and the shah of Iran and Henry A. Kissinger encouraged the Kurdish rebellion against the Iraqi government. C.I.A. agents were sent to the Iraq-Iran border to help the Kurds.
The Kurdish leader Mustafa Barzani did not trust the shah of Iran but believed Kissinger when he said that the Kurds would receive help from the Americans.
But a year later, the shah of Iran made a deal with Saddam Hussein on the sidelines of an OPEC meeting: In return for some territorial adjustments along the Iran-Iraq border, the shah agreed to stop support for the Kurds.
Kissinger signed off on the plan, the Iraqi military slaughtered thousands of Kurds and the United States stood by. When questioned, Kissinger delivered his now famous explanation: “Covert action,” he said, “should not be confused with missionary work.”
In the fight against ISIS in Syria, Kurdish fighters followed their hard-fought triumph in Kobani by liberating other Kurdish towns. Then the Americans asked their newfound Kurdish allies to go into Arab areas, team up with local militias and reclaim those areas from the Islamic State.
The American military implored the S.D.F. to fight in the Arab areas, and so they advanced, seizing Raqqa and Deir al-Zour, winning but suffering large numbers of casualties.
The American-Kurdish military alliance against the Islamic State in Syria and Iraq “began with us helping them,” said Peter W. Galbraith, the former American diplomat who has for years also been a senior adviser to the Kurds in both Syria and Iraq. “But by the end, it was them helping us. They are the ones who recovered the territory that ISIS had taken.”