In early morning tweets, Trump said he was fulfilling a promise from his 2016 presidential campaign to leave Syria. Echoing a central plank of his foreign policy - that he seeks to stop the United States being taken advantage of - he said the United States was doing the work of other countries, including Russia and Iran, with little in return.
"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever? Time for others to finally fight," he tweeted.
The United States is also ending its air campaign against the militants in Syria, US officials told Reuters on Thursday.
Trump had pledged to withdraw from Syria if elected, but the abrupt announcement on Wednesday to withdraw the roughly 2,000 US troops from Syria caught US lawmakers and others by surprise. The White House declined to offer a timeline for the departure.
Some of Trump's fellow Republicans strongly criticized the move, saying they were not briefed ahead of time and that the move strengthened the hand of Russia and Iran, which both support Syrian President Bashar al Assad.
US officials, speaking on condition of anonymity, have told Reuters that US commanders on the ground are also concerned about the impact of a quick withdrawal and were surprised by the decision.
Trump's move also drew criticism from some US allies, including Britain and France, which said Islamic State militants had not been defeated and that its troops would remain in Syria.
The void left by the US withdrawal could undermine diplomatic efforts to end Syria's eight-year civil war and undercut the United States' leverage in the region as well as leave it few options if Islamic State surges again.
"ISIS hits us they are doomed!" Trump tweeted, using an acronym for Islamic State.
Islamic State, which declared its so-called caliphate in 2014 after seizing large areas of Syria and Iraq, was down to its last 1 percent of the territory it once held in Syria and has no territory in Iraq, a senior US official last week. But US officials have warned that retaking territory is not the same as defeating the militants.
In a campaign to defeat the group in Syria, former President Barack Obama ordered air strikes from September 2014 and then troops into the country the following year.
Some US lawmakers, including some Democrats, backed Wednesday's announced withdrawal but said Congress needed to play a larger role in overseeing such military operations.
Russian President Vladimir Putin on Thursday welcomed Trump's decision and said he largely agreed that Islamic State had been defeated.
Trump, however, said Russia - as well as Iran and Syria - were unhappy about the US withdrawal "because now they will have to fight ISIS and others, who they hate, without us."
Democratic US Senator Chris Murphy said Trump's statements appeared in conflict: "Wait a minute... I thought we defeated ISIS. Why would Russia, Iran and Syria have to fight them if they are defeated?" he tweeted in reply to Trump.