WASHINGTON -- President Donald Trump is pulling all 2,000 U.S. troops out of Syria, officials announced Wednesday as the president suddenly declared victory over the Islamic State, contradicting his own experts' assessments and sparking surprise and outrage from his party's lawmakers who called his action rash and dangerous.

The U.S. began airstrikes in Syria in 2014, and ground troops moved in the following year to battle the Islamic State, or ISIS, and train Syrian rebels in a country torn apart by civil war. Trump abruptly declared their mission accomplished in a tweet.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," he said as Vice President Mike Pence met with top leaders at the Pentagon. U.S. officials said many details of the troop withdrawal had not yet been finalized, but they expect American forces to be out by mid-January.

Later Wednesday, Trump posted a video on Twitter in which he said is "heartbreaking" to have to write letters and make calls to the loved ones of those killed in battle. "Now it's time for our troops to come back home," he said.

A senior administration official, speaking to reporters on condition of anonymity, said Trump made the decision based on his belief U.S. troops have no role in Syria beyond combatting Islamic State, whose fighters are now believed to hold about 1 percent of the territory they did at the peak of their power.

The president informed Turkish President Recep Tayyip Erdogan of his decision in a telephone call, the official said. Turkey has recently warned it would launch combat operations across its southern border into northeastern Syria against Kurdish forces who have been allied with the U.S. in the fight against the Islamic State.

Trump's declaration of victory was far from unanimous, and officials said U.S. defense and military leaders were trying to dissuade him from ordering the withdrawal right up until the last minute. His decision immediately triggered demands from Congress, including leading Republicans, for more information and a formal briefing on the matter. Sen. Lindsay Graham of South Carolina, just returned from Afghanistan, said he was meeting with Defense Secretary Jim Mattis late in the day.

Graham, typically a Trump backer, said he was "blindsided" by the report and called the decision "a disaster in the making." He said, "The biggest winners in this are ISIS and Iran."

The decision will fulfill Trump's long-stated goal of bringing troops home from Syria, but military leaders have pushed back for months, arguing the IS group remains a threat and could regroup in Syria's long-running civil war. U.S. policy has been to keep troops in place until the extremists are eradicated.

The senior administration official said American forces would still work with allies to fight the Islamic State or other extremists in the country but gave no details on what that might entail.

Another official said it still is not clear to defense leaders whether U.S. airstrikes against IS insurgents will continue in Syria after the American troops leave. U.S. military officials worry American-backed Kurdish troops will be targeted by Turkey and the Syrian government, leaving no ally on the ground to help direct the strikes.

Israeli Prime Minister Benjamin Netanyahu, who remains concerned about Iranian efforts in the area, reacted in noncommittal fashion after talking with Trump by telephone.

"This is, of course, an American decision," he said. No matter what, he said, "we will safeguard the security of Israel and protect ourselves from this arena."

Leading Republican senators reacted with displeasure to the news.

Sen. Marco Rubio of Florida said the withdrawal would be a "grave error" and Kurdish fighters will stop fighting the Islamic State when they must confront Turkish troops crossing the border into Syria.