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 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.

A senior administration official, speaking on condition of anonymity, said Trump made the decision based on his belief that U.S. troops have no role in Syria beyond combating the Islamic State, whose fighters now hold about 1 percent of the area 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 that 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 for more information and a formal briefing on the matter. Sen. Lindsay Graham of South Carolina, 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.”

The decision will fulfill Trump’s long-stated goal of bringing troops home from Syria, but military leaders have pushed back for months, arguing that Islamic State remains a threat and could regroup in Syria’s long-running civil war.