Senate Republicans uncharacteristically lashed out at President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Wednesday for announcing a sudden and immediate withdrawal of U.S. troops from Syria, a decision that came without consulting Congress and seemed to catch the Pentagon off guard.

Several lawmakers said Congress received no advance notice of Trump’s announcement, leaving them fuming and scratching their heads.

“I don’t know what they’ve done, but this is chaos,” Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.), a staunch Trump ally and Armed Services Committee member, told reporters, adding that he planned to discuss the matter with Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE.

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Shortly before that, Graham said in a statement that a troop withdrawal would be an “Obama-like mistake,” a jab sure to catch the attention of a president driven in part by diverging from his predecessor.

On Wednesday morning, Trump declared victory against ISIS in Syria in a tweet after The Wall Street Journal, followed by The Washington Post, reported that he was preparing an immediate withdrawal of U.S. troops from Syria.

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,” Trump tweeted.

The Pentagon had no immediate comment on the news reports or Trump’s tweets, other than to say the military campaign was continuing “at this time.”

The White House later confirmed in a statement the administration has "started returning United States troops home" but emphasized coalition efforts in Syria would proceed. It also reiterated Trump's claim that "the United States has defeated the territorial caliphate."

"The United States and our allies stand ready to re-engage at all levels to defend American interests whenever necessary, and we will continue to work together to deny radical Islamist terrorists territory, funding, support, and any means of infiltrating our borders,” White House press secretary Sarah Huckabee Sanders said in a statement.

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Hours later, the Pentagon released a four-sentence statement from the chief spokeswoman, saying the military has “started the process of returning U.S. troops home from Syria as we transition to the next phase of the campaign.”

“The coalition has liberated the ISIS-held territory, but the campaign against ISIS is not over,” spokeswoman Dana White said in the statement. “For force protection and operational security reasons, we will not provide further details.”

In addition to ongoing military options against the Islamic State in Iraq and Syria (ISIS), those who favor keeping troops in Syria argue that a withdrawal would cede influence in the Middle East to Iran and Russia.

“It’s a terrible decision,” Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (R-Fla.) said Wednesday. “I hope it can be entirely or at least partially reversed, or we’re going to pay a big price for it in the years to come.”

National security adviser John Bolton called some senators after the news broke Wednesday, with Senate Armed Services Committee Chairman James Inhofe (R-Okla.) and Senate Majority Whip John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas) saying they were among those who received calls. A senior administration official said on a background call with reporters that the number of lawmakers called by the afternoon was “large.”

Cornyn said senators were “surprised” at the announcement.

Inhofe said he and his committee “should have known” and expressed concern about protection for U.S.-backed Kurdish forces going forward.

Asked about Trump’s assertion of victory over ISIS, Inhofe paused, before saying, “we’ve made great strides.”

Vice President Pence took the brunt of GOP ire during lunch at the Capitol. Outgoing Senate Foreign Relations Committee Chairman Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) said Pence faced “tough questions” during the lunch.

Corker later went to the White House to meet with Trump, but the meeting was canceled while he was there and instead he spoke with Bolton.

Returning to the Capitol, Corker called Trump’s decision “in many ways even worse” than Obama’s withdrawal from Iraq that critics say allowed for the growth of ISIS. Corker accused Trump of abandoning the U.S.-backed Syrian Democratic Forces (SDF) “six to eight weeks” away from clearing ISIS from its territory.

“I doubt there’s anybody in the Republican Caucus in the Senate that just isn’t stunned by this precipitous decision,” Corker said. “My sense is that it’s been a shock throughout the administration that this type of decision was made. … It’s hard to imagine that any president would wake up and make this kind of decision with this little communication, with this little preparation.”

He added that based on conversations with people close to the decision, he does not see the decision being reversed and that Trump has “made his mind up.”

Corker also said that “to his knowledge” allies in the coalition fighting ISIS also were not consulted ahead of the announcement.

In the background call with reporters on Wednesday afternoon, the senior administration official had no details on a timeline for withdrawal, how many troops have already left or whether the military would continue airstrikes in Syria, referring questions to the Pentagon.

The official said the withdrawal would be done in an “orderly fashion,” adding the Pentagon was “working [on the timeline] right now.”

Reports citing unnamed officials said State Department personnel will withdraw from the country as well. A department spokesperson declined to comment, citing “operational security reasons.”

The State Department canceled its regularly scheduled press briefing for Wednesday.

Despite Trump’s declaration of victory, independent analyses and lawmakers say ISIS continues to hold territory in Syria.

“ISIS is not defeated in Syria. It controls territory, retains significant fighting forces and is reconstituting those forces,” the Institute for the Study of War tweeted Wednesday, along with its map of ISIS’s areas of control, attack and support as of mid-November.

The United States has about 2,000 troops in Syria backing local forces fighting ISIS.

Troops were first deployed to Syria in October 2015.

During his presidential campaign, Trump pledged to “bomb the shit” out of ISIS and then quickly bring troops home, chastising what he saw as years of failed nation-building missions in the Middle East and elsewhere.

His advisers, though, had so far persuaded him to stay the course on U.S. military operations around the world. In Afghanistan, Trump agreed after months of deliberations to send thousands more troops and keep them there indefinitely.

In Syria, Trump first suggested an imminent withdrawal in March. During a speech about infrastructure, he made a seemingly off-the-cuff remark that the United States will “be coming out of Syria, like, very soon.”

Days later, he reiterated, “I want to get out. I want to bring those troops home.”

But then Trump agreed in April to leave U.S. troops there until ISIS was defeated, though he was said to have given the military a six-month timeline to do so.

After that, administration officials including Bolton and special envoy for Syria engagement James Jeffrey have talked about an indefinite troop commitment. In September, Bolton pledged troops would not leave until Iranian forces do.

Operations in the past few months have focused on routing ISIS from its remaining pockets in the Middle Euphrates River Valley. On Friday, the SDF declared victory over ISIS in Hajin, the last major town held by the terrorist group, but several villages and other areas remain under ISIS control.

Last week, the administration’s special envoy for the anti-ISIS coalition, Brett McGurk, characterized a hypothetical withdrawal as “reckless.”

“Obviously, it would be reckless if we were just to say, ‘Well, the physical caliphate is defeated, so we can just leave now.’ I think anyone who’s looked at a conflict like this would agree with that,” McGurk, an Obama administration holdover, told reporters.

Trump’s decision comes after Turkish President Recep Tayyip Erdoğan threatened to launch a new assault against U.S.-backed Kurdish forces in Syria. Those forces make up the bulk of the SDF, but Ankara considers them terrorists connected to Kurdish insurgents in Turkey.

On Monday, Erdoğan said he got “positive answers” from Trump during a Friday phone call on Syria.

In the call with reporters Wednesday, the administration official denied Erdoğan swayed Trump’s decision.

“The president made his own decision,” the official said.

In its Wednesday statement on Syria, coalition member and close U.S. ally the United Kingdom noted that “much remains to be done, and we must not lose sight of the threat they pose.”

“Even without territory, Daesh will remain a threat,” the statement said, using an Arabic name for ISIS.

Trump received some praise from those who have never supported the U.S. military deployment in Syria.

“I am happy to see a President who can declare victory and bring our troops out of a war,” Sen. Rand Paul (R-Ky.) tweeted. “It’s been a long time since that has happened.”

Still, some lawmakers opposed to the deployment questioned Trump’s larger Syria strategy in the wake of the withdrawal.

“I’m glad to hear the troops are coming back, but it’d be nice to hear what the overall strategy is,” Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.) said. “I just have no idea what his plan is. When he had troops there at least I knew that he had some long-term plan. If we move the troops out, are we putting more State Department personnel in? Are we putting more development dollars into the areas where you have some American interests at stake?”