"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 real estate mogul turned president, returning to his pre-White House isolationism, on Wednesday ordered the pullout of the 2,000 US troops in a decision that caught off-guard US allies, the Pentagon and members of Congress, several of whom attempted to persuade Trump to reconsider.
Secretary of State Mike Pompeo, who like most US officials had been speaking for months about a long-term commitment in Syria, promised that the fight was not over against the Islamic State extremists, also known as ISIS.
"The United States of America intends to continue that counterterrorism campaign, continue the fight against ISIS, whether it stems from Syria or other places," Pompeo told the conservative radio host Laura Ingraham.
The United States will keep up air strikes in Syria so long as troops remain, Pentagon spokeswoman Commander Rebecca Rebarich said, but it was unclear what would happen afterward.
A US defense official told AFP that "there's not a lot of clarity on the future situation," with the air campaign contingent on what allies do.
Putin praises Trump
France and Britain both have small contingents of special forces inside Syria, focused like the US troops on battling the Islamic State group.
The US withdrawal will make Russia, which has deployed its air power in support of President Bashar al-Assad, the pre-eminent global power in the conflict.
As Trump tweeted that Russia wanted US forces to stay and blamed the "fake news" media for reporting otherwise, Putin publicly rejoiced over the pullout order.
"The fact that the US has decided to withdraw its troops is right," Putin said during an annual year-end press conference, saying that "on the whole I agree with the US president" on the level of damage inflicted on Islamic State.
Putin, who has described the fall of the Soviet Union as a historic geopolitical disaster, sees Moscow's longtime ally Syria as a key asset in preserving influence in the Middle East.
Iran's Shiite clerical regime has also strongly backed Assad, a secular leader from the heterodox Alawite sect.
Turkey opposes Assad and may be emboldened by Trump to attack Kurdish fighters inside Syria, who fought alongside US troops against the Islamic State group.
Turkey links the Kurds who dominate the US-backed Syrian Democratic Forces to a decades-old insurgency at home, but had been reluctant to strike for fear of setting off a crisis if the United States suffered casualties.
Mustefa Bali, a spokesman for the Syrian Democratic Forces, said the fighters would keep up the battle against Islamic State - but that all bets were off if Turkey attacks.
"The scenario of a halt in the anti-terrorist battle is tied to Turkish threats," he told AFP.
Worries in Europe
Bali said the Kurdish forces would keep locked up the Islamic State extremists in their custody -- but alleged that Turkey may target prisons to sow chaos once US troops leave.
French President Emmanuel Macron's office called the prisoner issue "extremely important for France" and urged discussions to avoid "one of the negative consequences of a hasty retreat."
The Islamic State movement has claimed credit for a slew of bloody attacks around the world, including the 2015 coordinated assault on Paris, and experts estimate that thousands of sympathizers remain.
Germany, which has taken in more than one million refugees stemming in large part from the Syria conflict, questioned Trump's assessment that the threat was over.
"There is a danger that the consequences of (Trump's) decision could hurt the fight against the IS and endanger what has been achieved," German Foreign Minister Heiko Maas said in a statement.
While fighting has largely subsided in Syria and the Islamic State group holds little territory, a political solution remains elusive in ending the war that has killed more than 360,000 and displaced millions since 2011.
The outgoing UN envoy to Syria, Staffan de Mistura, acknowledged Thursday to the Security Council that a goal would not be met of setting up a committee by the end of the year to write a new Syrian constitution.
James Jeffrey, the US pointman on Syria, had been due to take part in the New York meeting. A State Department official said that Jeffrey instead stayed in Washington "given recent developments."