The New York Times reports Pentagon officials, including Defence Secretary James Mattis, were still trying to talk Trump out of his decision hours before it became public knowledge.
They argued America would be abandoning its allies, the Kurds, and ceding influence in the region to Russia and Iran.
The President's political allies, who can normally be relied upon to defend even his most controversial actions, have gone public with their shock.
"This is a terrible mistake," Republican Senator Marco Rubio said.
"It will have grave consequences for the US and Israel, and great benefit for ISIS, Iran and Hezbollah."
Rubio said the "hasty" withdrawal would cause America's allies in Syria, the Kurds, to end their fight against Islamic State and "turn Syria over to Israel's greatest enemies".
"The decision to pull out of Syria was made despite overwhelming military advice against it. It is a major blunder. If it isn't reversed it will haunt this administration and America for years to come," he said.
Another Republican, Congressman Adam Kinzinger, told Fox News he was "blown away" by Trump's decision and it would be a "devastating blow" in the war against Islamic State.
Trump spoke with Turkey's President Recep Tayyip Erdogan last week. That conversation could be behind his decision to withdraw US troops from Syria. Photo / AP
"I want to support President Trump in every way I can. But on something like this, you have to look at this and go, 'This is going to be devastating for foreign policy.' And I don't even think we begin to know the ramifications," he said.
"They're celebrating in Moscow and Tehran right now. They're trembling in Tel Aviv. And in Washington we're confused, because nobody saw this coming."
Senator Lindsey Graham was equally scathing. He said he had been "blindsided" by the withdrawal, and called it a "huge Obama-like mistake".
"With all due respect, ISIS is not defeated in Syria, Iraq, and after just returning from visiting there, certainly not Afghanistan," Graham said.
"An American withdrawal will put the Kurds and all those who came to America's aid in destroying ISIS at tremendous risk. It will make it more difficult to recruit future partners willing to confront radical Islam.
"If Obama had done this, all of us would be furious. If Obama had done this, we'd be going nuts right now."
Graham indicated he would demand congressional hearings on the decision.
Barack Obama faced similar criticism when he withdrew US forces from Iraq in 2011. Islamic State took advantage of the power vacuum he left behind.
A US soldier waves as he sits on an armored vehicle, at a road leading to the tense front line with Turkish-backed fighters, in Manbij town, north Syria. Photo / AP
And George W. Bush ended up ruing his declaration of "mission accomplished" in Iraq in 2003.
There are a few Republicans still defending Trump.
"I am happy to see a President who can declare victory and bring our troops out of a war. It's been a long time since that has happened," Senator Rand Paul said.
The President promised to withdraw from Syria during the 2016 election campaign. But why do it now? The answer may lie with Turkey, which borders Syria to the north.
Turkish President Recep Tayyip Erdogan has been threatening to launch a military incursion into northeast Syria, which is under Kurdish control. He considers the Kurds — who US forces have armed and worked alongside in the fight against Islamic State — a terrorist group allied with separatists in Turkey.
Trump, who spoke to Erdogan on the phone last week, reportedly feared American troops could be caught in the crossfire.
After historic victories against ISIS, it’s time to bring our great young people home! pic.twitter.com/xoNjFzQFTp
"We've been fighting for a long time in Syria. I've been President for almost two years, and we've really stepped it up, and we have won against ISIS," he said in a video elaborating on his reasons for withdrawing.
"We've beaten them, and we have beaten them badly, and we've taken back the land. And now it's time for our troops to come back home."
Trump said he found it "heartbreaking" to write letters to the parents of fallen soldiers.
"Now we've won. It's time to come back. They're getting ready; you're going to see them soon."
So has Islamic State truly been defeated? Not quite.
The terror group has lost the vast majority of the territory it once held, and 2500 remaining fighters have been pushed back to a narrow strip of land along the border of Syria and Iraq.
"Walking away now is a remarkable gift for ISIS, whose leaders can say they saw off the Americans and their allies. Even without the withdrawal, the group would likely have held out for many more months," the Guardian's Martin Chulov said.
"With its most formidable foe leaving the fray, ISIS may well be reborn."
Trump's decision has also surprised other parts of his own administration.
On December 11, the State Department's special envoy for defeating Islamic State, Brett McGurk, warned a media briefing the fight against Islamic State could not end yet.
"Very clearly, the military objective is the enduring defeat of ISIS. And if we've learned one thing over the years, enduring defeat of a group like this means you can't just defeat their physical space and then leave," he said.
"You have to make sure the internal security forces are in place to ensure that those gains, security gains, are enduring.
"That will take some time."
During the 2016 campaign, Trump berated Barack Obama for pulling out of Iraq "too early" without considering the consequences.
"Here he is ignoring precisely that lesson," David Adesnik, a Syria analyst at the Foundation for Defence of Democracies, told CNN.
"Every indicator that would tell you this is a premature withdrawal is back again."
