Washington (CNN) President Donald Trump has ordered staff to execute the "full" and "rapid" withdrawal of US military from Syria, declaring that the US has defeated ISIS.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," Trump tweeted Wednesday morning. Planning for the pullout is already underway, a US defense official and an administration official told CNN.

The decision, a sharp reversal from previously stated US policy, surprised foreign allies and lawmakers, sparking rebukes, rebuttals and warnings of intensified congressional oversight, even as the White House said troops are already on their way home.

"I'm pretty annoyed," said Sen. Lindsey Graham, a South Carolina Republican and Trump ally, who told CNN's Manu Raju the President's decision was "Obama-like" -- a reference to President Barack Obama's decision to pull troops out of Iraq in 2011, which critics say gave rise to ISIS. Graham said the role of Congress is "to make administrations explain their policy, not in a tweet, but before Congress answering questions."

'Betrayed'

The President's decision flew in the face of policy statements by administration officials just days earlier and military statements about the threat of ISIS, highlighting the continuing dysfunction at the most senior levels of Trump's administration.

Even though the US will continue to maintain troops in Iraq with the capability of launching strikes into Syria, many analysts said a withdrawal of ground forces will please US enemies by clearing the way in Syria for the Assad regime, Russia and Iran. A US departure could leave allies questioning Washington's commitment, reduce US awareness of dynamics on the ground and diminish Washington's influence in the region.

It is "extraordinarily shortsighted and naive," said Charles Lister, a senior fellow at the Middle East Institute, who added that the decision will not only leave Iran hawks -- including lawmakers and Cabinet members such as national security adviser John Bolton and Secretary of State Mike Pompeo -- feeling "betrayed," but also plant the seeds for rebellion among Republican ranks.

"This is a specifically Trumpian decision, and one that will be deeply unpopular within the vast majority of the GOP's foreign policy machine," Lister said. "Whether it takes hours or months, we will see some serious resistance coming out of this."

The fallout for Iran policy will be significant, said Derek Chollet, a US assistant secretary of defense in the Obama administration and now an executive vice president at the German Marshall Fund. "This drives a stake into the heart of the administration's Iran strategy."

Ignoring the lesson

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The churn in the administration's foreign policy echoes -- and may be designed to distract from -- major upheaval on the domestic front, as Trump contends with criminal investigations into his campaign, transition, inaugural committee and presidency, the dissolution of his charity amid charges he used it to enrich himself and his former national security adviser Michael Flynn awaiting sentencing after lying to the FBI about his contacts with Russian officials.

While Trump repeatedly returned to the idea of pulling troops out of Syria on the campaign trail and in office, the move is hard to square with his other policy priorities or his past criticism of Obama.

"Trump himself said on the campaign trail that he may not have liked being in Iraq, but Obama ruined a lot by pulling out too early and not thinking about what would happen next," said David Adesnik, the director of research and a Syria analyst at the Foundation for Defense of Democracies. "Here he is ignoring precisely that lesson."

"Every indicator that would tell you this is a premature withdrawal is back again," Adesnik added.

Lister said it was "extraordinary to see (Trump) repeating the same mistake and I actually think this could be worse," as Syria is in far worse shape than Iraq had been.

Allies blindsided

US allies in the region were blindsided by the announcement. Two diplomatic sources from two countries in the region said they had not been consulted or informed and that news of the planned withdrawal came as a "total surprise."

Tobias Ellwood, a minister in the British Ministry of Defense, said in a tweet that he "strongly" disagrees with Trump's comment on Wednesday that ISIS had been defeated. "It has morphed into other forms of extremism and the threat is very much alive," Ellwood wrote, while the Defense Ministry told CNN there would be no immediate change to its current operation in Syria.

According to a statement from Israeli Prime Minister Benjamin Netanyahu's office, Trump informed Netanyahu on Monday of his decision. Netanyahu also had a conversation Tuesday with Pompeo. The two men reassured the Prime Minister that the US had "other ways of expressing their influence in the area," the statement added.

Iran is Israel's central concern. In September, Bolton said the United States wouldn't leave Syria as long as Iranian forces continued to operate there, directly linking any withdrawal of American troops to the departure of Iranian forces.

"We're not going to leave as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias," Bolton said at the time.

'It's a big reversal'

Asked if Wednesday's announcement meant that threat no longer exists, a White House official who declined to be identified publicly told CNN, "US forces will continue the fight against ISIS. We will continue to use tools of national power, including economic sanctions and diplomatic pressure, as leverage to press for the withdrawal of Iranian-backed forces."

But the administration's ability to push back on Iran -- a major foreign policy priority -- will take a hit because of Trump's decision, Adesnik said.

"It's a big reversal," he told CNN. "We have announced a policy of pushing back against Iran's nuclear violations and aggression in the region on every front. And Syria is an absolutely central front for Iran; for them it's an indispensable ally."

"You can't have a counter Iran strategy if you just hand Syria back to them," Adesnik said, "and it's not clear to me if this withdrawal in any way accounts for that."

Lister said it amounts to giving up "all leverage and as part of a regional strategy of containing Iran, all of that becomes a joke. We've just told Iran and all of our regional allies we don't believe in sticking it out to achieve our foreign policy objectives."

"Iran will take this as a huge vote of confidence," he said.

Troops on the ground

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The US has about 2,000 troops in Syria, where they are primarily training local forces to combat ISIS. The US-backed and Kurdish-led Syrian Democratic Forces have had some recent success against the terror group and are on the cusp of capturing the last major town held by ISIS east of the Euphrates.

"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," White House spokeswoman 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 said, adding that the US victories didn't signal an end of the global coalition on ISIS or its campaign.

Estimates vary as to how many ISIS fighters are left in Syria, but Lister noted that "we're arguably years from achieving" a defeat of ISIS.

"To make that fact as clear as day," he said, only 10 minutes before Trump sent his Wednesday tweet, "ISIS claimed responsibility for an attack in Raqqa," a town the US had liberated and where US troops and US Agency for International Development and State Department personnel are deployed.

Threading the needle

The anti-ISIS coalition has estimated there are some 2,000 ISIS fighters in the town of Hajin, the terror group's last redoubt. But a Defense Department inspector general report put the number of ISIS members in Syria and Iraq as high as 30,000.

Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, said this month that the US needed to train thousands of local fighters to ensure a lasting defeat of the terror group

The chief Pentagon spokesperson, Dana W. White, tried to thread the needle. "The Coalition has liberated the ISIS-held territory, but the campaign against ISIS is not over," she said in a statement. "We have started the process of returning U.S. troops home from Syria as we transition to the next phase of the campaign. We will continue working with our partners and allies to defeat ISIS wherever it operates."

For force protection and operational security reasons, White said the Pentagon would not provide further details. A State Department spokesperson, speaking on background, said that for operational security reasons the department would have no comment on the movement of agency personnel.

This story has been updated.