Facing condemnation from Republican and Democratic lawmakers, Trump and senior U.S. officials tried to clean up the announcement by making clear that the U.S. does not support that kind of Turkish assault and saying the U.S. was simply moving troops out of potential harm's way.
Alarmed U.S. officials warned that the decision could empower the Islamic State and push the Kurdish forces who served as the foot soldiers for the U.S. and its coalition against ISIS to turn now to Syrian leader Bashar al-Assad and his backers, Russia and Iran.
"They abandoned us," Gen. Mazloum Ebdi, the commanding general of the Syrian Democratic Forces, told ABC News about Trump's decision through an interpreter.
The SDF are a majority Kurdish force that bore the brunt of the fighting against the terror group.
Some U.S. officials tried to downplay the decision, saying it was a small group of U.S. troops and that the president only intended to get them out of harm's way, not change U.S. policy. But a senior U.S. official told ABC News it could overturn years of effort to defeat ISIS.
"Everyone in the military from the youngest Green Beret in Syria to the secretary of defense is opposed to withdrawing our forces," the senior official said. "I don't think anyone advised this."
After his call with Erdogan on Sunday, however, the White House announced those troops would move out. Instead, while they "will not support or be involved in the operation," they also "will no longer be in the immediate area," White House press secretary Stephanie Grisham announced.
That was interpreted as a green light for Turkey to move forward with its offensive. Ebdi told ABC News that the U.S. "abandoned us in the middle of struggle against ISIS" and warned a Turkish offensive is going to lead to ethnic cleansing and change the demographic in the area. This is what the Turks are looking to do, he added.
By Monday afternoon, the administration was amending its statement to say they opposed a Turkish invasion and wanted to protect America's Kurdish allies.
"I told Turkey if they do anything that is not humane, Turkey will suffer a very decimated economy," Trump said at the White House. "If Turkey does anything that they are not supposed to do, we will hit them so hard in their economy."
But he argued U.S. forces there were "policing" and should come home. Last December, Trump said he was withdrawing all U.S. forces from Syria -- a process that started, but has slowed as the administration says it is seeking a "responsible and deliberate" withdrawal.
That is not happening imminently, according to a senior administration official, who told reporters Monday evening, "This is not the time for any such move."
There have been conflicting reports about how many U.S. troops have been pulled back. A senior State Department official told reporters the move affected two "very small" attachments of U.S. troops, about two dozen in total. Trump said it was about 50 troops, while Ebdi said it was about 150 in total.
The U.S. is also not withdrawing those forces from Syria, just pulling them back for now.
"At the moment, there is no decision to withdraw the rest of the U.S. forces from northeast Syria. We're reviewing the situation based upon the military situation," the State Department official said.
The U.S. has also sent a warning to Turkey about an assault by blocking them from using the airspace over northeastern Syria, according to the official.
But they will not use military force to stop an assault, the official added, even after senior U.S. officials have said they want to make sure the Turks don't "slaughter" the Kurds.
"It's important that we do everything we can to ensure that those folks that fought with us are protected," Secretary of State Mike Pompeo told ABC News in January.
Two months after the Pentagon inspector general warned that ISIS is resurgent in Syria, the decision to pull back forces may also risk empowering the terror group by undermining the Kurdish-run detention facilities where some 10,000 ISIS fighters are detained.
Kurdish guards "are going to head to the border area to defend their families" against a potential Turkish assault, Ebdi told ABC News. "This lack in security of course is going to jeopardize the calm in the prisons."
Senior U.S. officials, like ambassador-at-large for counter terrorism Nathan Sales, have warned that the detention facilities are already hotbeds of radicalization at risk of jailbreaks. The senior State Department official conceded the threat of strengthening ISIS is real, but said the U.S. warned Turkey it would be responsible for any prisoners who escaped.
Ebdi said Monday that the decision has "hurt the credibility" of the U.S., adding, "When you broke or hurt the credibility or the confidence, it's not an easy thing to recover it."
The senior U.S. official told ABC News, "We should have stood with our partners as they have stood with us."
ABC News's Sarah Kolinovsky contributed to this report.