President Donald Trump declared victory over ISIS in Syria on Wednesday, as the Pentagon made preparations for an immediate troop withdrawal.

'We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,' the president wrote on Twitter.

The announcement came under immediate scrutiny, as policymakers admitted they had been blindsided and worried that Trump was making a monumental misstep.

Allies of the president on Capitol Hill called it a 'huge Obama-like mistake' and 'great disservice' to the nation. One GOP congressman said he was 'speechless' and that Iran and Russia are probably 'cheering.'

The White House launched a defense of the policy on a conference call on Wednesday afternoon with reporters in which a senior administration official claimed that no one should be 'surprised' by the announcement, because Trump has been saying he wants to get out of Syria since he was a candidate.

President Donald Trump declared a victory in Syria in the U.S. campaign against ISIS as the military prepares for a total pullout

After declaring victory on Twitter, President Trump, who had no public schedule Wednesday, released a video from the White House where he said: 'We have beaten them and we've beaten them badly. We've taken back the land.'

Yet, the same official could not offer a timeline for the troops are actually leaving, saying it was was still under design by the Pentagon.

'It's not that I'm not telling you, it's that I don't know, quite frankly,' the senior official said.

The person directed all questions of that nature to the Department of Defense.

Trump declared victory in Syria over ISIS on Wednesday morning in a surprise announcement that the U.S. was pulling troops out of the region

GOP lawmakers were miffed that Trump declared victory against ISIS over Twitter before they had been notified of his intent to withdraw troops.

Vice President Mike Pence just happened to be on Capitol Hill for a weekly meeting with Republican senators on Tuesday and took the brunt of the criticism behind closed doors, CNN reported.

'It doesn’t feel to me there was any inter-agency process,' Senate Foreign Relations Chairman Bob Corker said of the conversation with the vice president. 'I’m not sure even the principles knew.'

Corker suggested that Trump just 'woke up' and decided to make the policy announcement.

He told reporters at the Capitol hours later that he was at the White House for a planned meeting with Trump when the president, who has not been seen all day, cancelled on him.

The Tennessee Republican is leaving Congress for good at the end of this year and is effectively a lame duck. His complaints were shared by other lawmakers, however, who begged Trump to rethink his position.

Texas Sen. John Cornyn, the GOP whip, said it was a 'mistake' to leave Syria and the matter would most certainly be investigated by the upper chamber.

'I think the first mistake was to surprise Members of Congress. Nobody likes to be surprised,' he told CNN.

Graham also chided Trump for announcing the withdrawal on Twitter.

In a tweet, he said, 'Withdrawal of this small American force in Syria would be a huge Obama-like mistake.'

'This is the role of the Congress. To make administrations explain their policy, not in a tweet, but before Congress answering questions,' he told reporters later.

Graham said he was 'annoyed' and pledged 'aggressive' oversight of the move.

Sen. Marco Rubio likewise said the withdrawal is a 'great disservice' to the nation and a 'big mistake' that invites 'America's adversaries America is an unreliable partner.'

'Today’s decision will lead to grave consequences in the months and years to come,' the Republican senator cautioned.

A senior administration official refused to get into the details of the 'deliberative process' a short time after on a call with reporters during which the White House said that National Security Advisor John Bolton had been calling lawmakers all day.

'He's very committed to keeping them informed,' the senior official said.

The official was left exposed on the call when a reporter brought up Bolton's September claim that the U.S. would not leave Syria until Iran's military and its proxies do.

'I think the issue here is that the president has made the decision, and so previous statements — he gets to do that, that's his prerogative,' the official said in an admission that Trump was undermining his own aides.

Responding directly to complaints that legislators were informed after the fact, the person said, 'The president's views have been well known, so I really don't see this as a surprise.'

The official said that the comparison that Graham and others were making to Obama's withdrawal of U.S. troop from Iraq in 2011 was 'apples to oranges,' because the drawdown was much larger and came about as a result of a failure to come to an agreement with the Iraqi government about leaving troops in the country.

'The president has judged that the purpose for which those troops have been put into Syria has been accomplished to the point where they can reposition,' the person said.

As for Sen. Graham, the official said 'he is entitled to his opinion' but it's unreasonable to think that the U.S. is going to turn 'Syria into a utopian democracy.'

Trump's declaration came after the Wall Street Journal and other outlets quoted Pentagon officials saying the U.S. would soon begin pulling out remaining forces, which number about 2,000.

'The Pentagon has an order to get to move troops out of Syria as quickly as possible,' a U.S. official told the paper.

The White House said in a statement following Trump's announcement that troops have 'started returning' home and a transition has begun.

'Five years ago, ISIS was a very powerful and dangerous force in the Middle East, and now the United States has defeated the territorial caliphate. These victories over ISIS in Syria do not signal the end of the Global Coalition or its campaign,' White House press secretary Sarah Sanders said in a statement.

'We have started returning United States troops home as we transition to the next phase of this campaign.'

She added, '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.'

Turkish president Recep Erdogan was likely to welcome the move. His country shares a border with Syria. So was Russia, which is allied with Syrian dictator Bashar al-Assad and is establishing a permanent military presence in the country as part of a deal with Damascus, and Iran.

Defense Secretary Jim Mattis is said to have pressed Trump not to pull out America troops out of concern that ISIS will rebound. It still controls some territory in region.

GOP senators expressed the same sentiments on Wednesday as they vented their frustrations.

GOP Rep. Adam Kinzinger disputed the president's claim that ISIS had been defeated

Graham compared Trump to his Democratic predecessor Barack Obama's resolve to pull troops out of Iraq, which critics say paved the way for ISIS to develop.

Illinois Rep. Adam Kinzinger also disputed the president's claim that ISIS had been defeated.

'This is simply not true,' Kinzinger wrote after retweeting the president.

He said in another message, addressing Trump's decision to pull out troops, 'Really? Iran is rejoicing right now. We left Iraq, and had to come back. I would sure hope the President and his advisers are smarter than this.'

Kinzinger said on CNN later that he was 'speechless' and that terrorists would be 'emboldened' by the withdrawal. 'This makes no sense to me,' he said. 'I don't get it.'

He said that Moscow and Tehran are 'cheering' over the decision he claimed makes America less safe.

House Foreign Affairs Chairman Ed Royce said he too was 'disturbed,' because while 'ISIS is on the run, it’s not yet defeated.'

'The limited number of U.S. forces in Syria is critical to rooting out these terrorists and stopping Iran from moving fighters and missiles to Israel’s doorstep. Our presence is also key to helping leverage a political transition to end Syria’s civil war,' he contended.

'The last administration showed what happens when arbitrary political deadlines – rather than reality on the ground – dictate policy in war zones. We must learn from the mistakes of the past, not repeat them.'

Retired Gen. Mark Hertling also turned in a devastating review in an appearance on CNN, saying the move appeared to be 'transactional' in nature, having to do with an arrangement with Turkey and Russia.

'This was very surprising to me—not only the announcement but the timing. I don't understand why it is happening now,' he told the network.

'The U.S. Forces have just started an offensive with the Syrian defense forces and the Kurds against the town of Hajin, which is considered an ISIS stronghold… You're talking about making both the Russians, the Syrian Bashar Al Assad, the Iranians and the Turks very happy about this move because the Kurdish forces are going to see this as us turning against them one more time.'

Hertling added: 'What is interesting that has not been brought up yet is there seems to be a transactional element to this, too,' Raw Story reported. 'Over the last days the U.S. Has sealed the deal with Turkey for $3.5 billion worth of Patriot missile systems that were signed yesterday by our State Department. I'm sure that played a role, a little bit, in this.'

This April 2, 2018 photo shows a general view of a US military base in the al-Asaliyah village, between the city of Aleppo and the northern town of Manbij. The United States is preparing to withdraw its troops from Syria, a major move that throws into question America's role in the region

Syrian Democratic Forces and U.S. troops are seen during a patrol near Turkish border in Hasakah, Syria November 4, 2018

The State Department subsequently cancelled its daily briefing with reporters. The White House held a call for journalists but neither Trump nor his spokeswoman appeared before cameras.

Just last week the Special Presidential Envoy for the Global Coalition to Defeat ISIS, McGurk, said at a State Department briefing that ISIS had not been defeated.

'Even as the end of the physical caliphate is clearly now coming into sight, the end of ISIS will be a much more long-term initiative. We’ve talked about that many times. Nobody working on these issues day to day is complacent. Nobody is declaring a mission accomplished,' he'd said. 'Defeating a physical caliphate is one phase of a much longer-term campaign.'

He said that ISIS was down to one percent of controlled territory and said the coalition was shifting its focus to how to sustain the gains it had made in the region.

'We’re prepared to maintain the stabilization effort in Iraq and Syria,' he said.

A U.S. Air Force munitions team member uncases the laser-guided tip for a 500 pound bomb to be loaded onto an unmanned aerial vehicle (UAV), for airstrikes on ISIL targets on January 8, 2016 at a base in the Persian Gulf Region

U.S. troops patrol near Turkish border in Hasakah, Syria, November 4, 2018

Posted on Aug. 27, 2014, by the Raqqa Media Center of the Islamic State group, a Syrian opposition group, which has been verified and is consistent with other AP reporting, shows a fighter of the Islamic State group waving their flag from inside a captured government fighter jet following the battle for the Tabqa air base, in Raqqa

Trump has said for months that he wants to 'get out' of Syria and would recall troops as soon he was able. His announcement on Wednesday that it had already started to happen, however, is what startled lawmakers.

The U.S. was believed to have about 2,000 troops in Syria, where Trump led a three-party coalition that dropped bombs on Assad's forces for illicit use of chemical weapons.

Some 5,200 troops are still believed to be across the border in Iraq, a number that the Pentagon has declined as a matter of routine since Trump took office to confirm.

Trump has derided Barack Obama for withdrawing U.S. forces from Iraq. Republicans claim that the move left a power vacuum in the country that ISIS took advantage of.

He in 2016 contended repeatedly that Obama was the 'founder of ISIS' and claimed he was the terror group's 'most valuable player.'