American-backed Kurdish-led fighters in northeast Syria accused President Trump of a “stab in the back” on Monday hours after the White House announced U.S. troops would vacate the area, giving a green light to Turkey to begin a long-awaited military operation against the group.

Mr. Trump’s surprise announcement late Sunday evening came as a shock to the U.S.-backed Syrian Democratic Forces (SDF), which have partnered with Washington for years in the battle against the Islamic State. The U.S. withdrawal from the area clears the way for a military assault by Turkey, which views the Kurdish-led SDF as a threat to its own security.

“There were assurances from the United States of America that it would not allow any Turkish military operations against the region,” said SDF spokesman Kino Gabriel, according to Reuters. “But the [president’s] statement today was a surprise and we can say that it is a stab in the back for the SDF.”

Local media reported Monday that U.S. forces had begun leaving northeast Syria. Turkish President Recep Tayyip Erdogan also said Monday that the withdrawal had begun.

In a statement early Monday afternoon, the Pentagon did not offer details on the U.S. exit but said it opposes any looming military action by Turkey against the SDF.

“The Department of Defense made clear to Turkey — as did the President — that we do not endorse a Turkish operation in Northern Syria. The U.S. armed forces will not support, or be involved in any such operation,” said Pentagon spokesman Jonathan Hoffman. “In conversations between the Department and the Turkish military we have consistently stressed that coordination and cooperation were the best path toward security in the area.”

Mr. Trump’s decision immediately sparked a harsh backlash on Capitol Hill, including by some of the president’s closest Republican allies.

Sen. Lindsey Graham, South Carolina Republican, said on Twitter that he’s likely to introduce a Senate resolution the opposing the decision and asking for it to be reversed.

“If press reports are accurate this is a disaster in the making,” he said, adding that the move will fuel an Islamic State comeback, could force the Kurds to align with Syrian dictator Bashar Assad and with Iran, and would be a “stain on America’s honor for abandoning the Kurds.”

Rep. Liz Cheney, Wyoming Republican, called the withdrawal a “catastrophic mistake.”

“Pulling out of northern Syria ignores that painful lesson, represents an abandonment of our Kurdish allies despite their vital contributions to the fight against ISIS, emboldens Iran, and serves as an undeserved gift to the Ergodan regime, which has only continued its steady march toward Moscow,” she said in a statement.

Sen. Tim Kaine, Virginia Democrat, blasted not only the policy decision itself but also the fact that the president apparently made the move without consulting congressional leaders.

“President Trump’s decision to pull U.S. troops from Syria’s border with Turkey and abandon the Kurds is a betrayal of a key partner in our fight against ISIS,” Mr. Kaine said in a statement. “Now they are on their own — facing threats from the Syrian government, Turkey, Russia, and ISIS remnants. And Trump took this step against the advice of our diplomats and military leaders. He didn’t even notify the Kurds, our allies, or Congress.”

The withdrawal from Syria makes good on a promise Mr. Trump made late last year, when he vowed to an “endless war” in the Middle East and bring troops home. The move led directly to the resignation of then-Defense Secretary James Mattis and sparked a fierce backlash on Capitol Hill.

Since then, however, the White House and Pentagon seemingly have slow-walked the withdrawal, and American forces have remained stationed in Syria.

Sunday evening’s announcement seems to change that. In a statement, the White House said that the president spoke with Mr. Erdogan on Sunday night and the two men discussed the situation inside Syria.

“Turkey will 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,” White House press secretary Stephanie Grisham said in a statement.

Regional analysts condemned the move as “insanity” and said it sends a horrible signal to U.S. allies.

“That we should hand over crucial real estate to Turkey, a country that now works against American interests in almost every way, borders on insanity,” Jonathan Schanzer, senior vice president for research at the Washington-based think tank Foundation for Defense of Democracies, said on Twitter. “And the idea of green lighting a war *against our own allies* is mind boggling.”

The White House also said it is passing responsibility to Turkey for all of the Islamic State fighters captured in recent years.

“The United States Government has pressed France, Germany, and other European nations, from which many captured ISIS fighters came, to take them back, but they did not want them and refused,” Ms. Grisham continued. “The United States will not hold them for what could be many years and great cost to the United States taxpayer. Turkey will now be responsible for all ISIS fighters in the area captured over the past two years in the wake of the defeat of the territorial ‘Caliphate’ by the United States.”

The Pentagon also shifted responsibility for captured Islamic State fighters to Turkey.

“As the president has stated, Turkey would be responsible, along with European nations and others, for thousands of ISIS fighters who had been captured and defeated in the campaign lead by the United States,” Mr. Hoffman said.

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