United States President Trump has ordered a rapid withdrawal of all 2,000 United States ground troops from Syria within 30 days, declaring the four-year American-led war against the Islamic State as largely won, officials said Wednesday.

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,” the president said in a Twitter post on Wednesday morning. He offered no details on his plans for the military mission, nor a larger strategy, in Syria.

Sarah Huckabee Sanders, the White House press secretary, said in a statement that “we have started returning United States troops home as we transition to the next phase of this campaign.”

But Pentagon officials who had sought to talk the president out of the decision as late as Wednesday morning argued that such a move would betray Kurdish allies who have fought alongside American troops in Syria and who could find themselves under attack in a military offensive now threatened by Turkey.