President Trump’s decision late Sunday to withdraw American troops from northern Syria has drawn an immediate backlash, with critics labeling the move a devastating betrayal of the U.S.’ Kurdish partners and a disaster in the battle against ISIS.

U.S. troops began withdrawing early Monday from Syria’s border with Turkey, following a phone call between Trump and Turkish President Recep Tayyip Erdogan.

U.S. forces had been supporting Kurdish fighters in Syria, which are stalwart allies in the battle against ISIS. Critics say the pullout green-lights a Turkish military assault on the Kurds, who Turkey views as terrorists, raising fears of potential ethnic cleansing.

It also raises serious concerns about the security of an estimated 80,000 ISIS prisoners currently held in Kurdish prisons. U.S. officials have previously warned the prisoners — about 12,000 fighters and their families — could form the basis of the next “caliphate.”

The move was met with outrage and disbelief from critics including Sen. Lindsey Graham (R-S.C.), a fervent Trump ally, who tweeted that the withdrawal was a “disaster in the making.”

He said the move would ensure an ISIS comeback and represented a “stain on America’s honor for abandoning the Kurds.”

The withdrawal started just hours after a statement from White House press secretary Stephanie Grisham late Sunday announcing Ankara would “soon be moving forward with its long-planned operation into Northern Syria.”

“The United States armed forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial ‘caliphate,’ will no longer be in the immediate area,” said the statement.

On Monday morning, Trump addressed the withdrawal in a series of tweets which, like the White House statement, pointedly blamed European nations for not having repatriated and prosecuted their nationals who are being held by the Kurds for alleged ISIS crimes, despite repeated requests from the U.S. and the Kurds.

“Europe did not want them back, they said you keep them USA! I said “NO, we did you a great favor and now you want us to hold them in U.S. prisons at tremendous cost,’” Trump tweeted, adding that Europe had been taking the U.S. for “suckers.”

His sole acknowledgment of the plight of the Kurds, who lost an estimated 11,000 lives in the U.S.-backed fight against ISIS, was that they “fought with us, but were paid massive amounts of money and equipment to do so.”

“It is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home,” he tweeted. “Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out, and what they want to do with the captured ISIS fighters in their ‘neighborhood.’”

The move represents a sudden policy swerve for the U.S., which had previously been attempting to reach a deal with both parties to prevent Turkey from invading Syria. Ankara views the Kurds as terrorists, indistinguishable from Kurdish separatists it is battling on its own soil.

The sudden pullout shocked the Kurdish authorities in Syria, who have relied on support from the United States for protection against many enemies in the region. In a series of tweets, the Kurdish-led Syrian Democratic Forces described how it had been removing fortifications and heavy weapons from along the border area in line with the U.S.-guaranteed agreement with Turkey.

“Erdogan's threats are aimed to change the security mechanism into a mechanism of death, displace our people & change the stable & secure region into a zone of conflict and permanent war,” it wrote on Twitter.

Brett McGurk, who was the U.S.’s Special Presidential Envoy to the global coalition against ISIS from 2015 until last year, was also scathing of the move, which he said “served up” Kurdish forces to Turkey.

“Donald Trump is not a Commander-in-Chief. He makes impulsive decisions with no knowledge or deliberation,” he tweeted. He warned Turkey’s plans for northern Syria would essentially extend its borders up to nearly 20 miles deep into Syria, making minorities such as Kurds and Christians vulnerable. He also said Turkey has “neither the intent, desire, nor capacity” to deal with the prisoners.

Lina Khatib, head of the Middle East and North Africa program at London’s Chatham House think tank, said it was difficult to envisage how the successful handover of ISIS prisoners from Kurdish to Turkish custody could take place, given the two parties were enemies. “All this does not bode well for Turkey’s prospects of managing the ISIS prisoners situation effectively,” she said.

Macer Gifford, an anti-ISIS campaigner who fought for Kurdish forces against the Sunni terror group, said the withdrawal had raised fears of ethnic cleansing for Kurds and other minorities like Syriac Christians from the region, especially given that Turkey was likely to make heavy use of its proxy forces, which contained many Islamist extremists.

Gifford called the move a gift to ISIS, which “thrives on chaos” and would be expected to try to liberate its supporters from Kurdish prisons and regroup. “ISIS has been waiting for this,” he said. “It’s going to be a nightmare.”