WASHINGTON — President Trump has begun what will be a total withdrawal of U.S. troops from Syria, declaring on Wednesday they have succeeded in their mission to defeat ISIS and were no longer needed in the country.

A decision to pull out completely, confirmed by U.S. officials, coincides with the roughly 2,000 U.S. troops finishing up a campaign to retake territory once held by ISIS militants.

News of a full withdrawal drew immediate criticism from some of Trump’s fellow Republicans, who said that leaving strengthened the hand of Russia and Iran, which both support Syrian President Bashar al Assad.

It may also leave exposed an alliance of Kurdish and Arab militias known as the Syrian Democratic Forces, or SDF, which has been among the most effective against ISIS but is under threat as Turkey weighs a new offensive in Syria.

U.S. commanders on the ground, who have developed strong ties to SDF leaders, had voiced concerns about what a fast withdrawal would mean for the U.S-backed forces and were surprised by the decision, according to U.S. official who spoke to Reuters on condition of anonymity.

President Trump

“We have defeated ISIS in Syria, my only reason for being there”. Trump tweeted on Wednesday

“We have started returning U.S. troops home as we transition to the next phase of this campaign,” White House spokeswoman Sarah Sanders said in a statement issued after Trump tweeted that “We have defeated ISIS in Syria, my only reason for being there.”

Trump’s predecessor, Barack Obama, was slow to get involved in Syria’s civil war, fearing being dragged into another open-ended foreign conflict like the one in Afghanistan. Hundreds of thousands of people have been killed and millions have been displaced in Syria with 22 million population.

But in a campaign to defeat ISIS in Syria, Obama ordered air strikes from September 2014 and then troops into the country the following year.

One U.S. official said Washington aimed to withdraw troops within 60 to 100 days and said the U.S. State Department was evacuating all its personnel in Syria within 24 hours. A second official said they could leave even sooner.

Trump is wary of open-ended war

Trump is wary of open-ended foreign conflicts and his decision on Syria raises questions about whether he may also reconsider the U.S. war effort in Afghanistan, where American forces have been fighting since 2001.

Trump reluctantly agreed to a troop increase last year, but U.S. officials have privately acknowledged a sense of urgency and are increasingly focused on securing a peace deal with a resurgent Taliban.

Some of Trump’s Republican allies in Congress railed against the pullout decision. U.S. Senator Lindsey Graham, often a Trump ally but generally a foreign policy hawk, said a withdrawal would have “devastating consequences” for the United States in the region and throughout the world.

U.S. troops started their complete withdrawal from Syria.

“An American withdrawal at this time would be a big win for ISIS, Iran, Bashar al Assad of Syria and Russia,” Graham said in a statement.

The surprise decision also raised eyebrows abroad.

A British defense minister said he strongly disagreed with Trump that ISIS had been defeated in Syria. Israeli Prime Minister Benjamin Netanyahu said Israel would study the decision and would ensure its own security.

In Russia, TASS news agency quoted the Foreign Ministry as saying withdrawing U.S. troops from Syria created prospects for a political settlement.

A complete withdrawal of U.S. troops from Syria would leave a sizeable U.S. military presence in the region, including about 5,200 troops across the border in Iraq. Much of the U.S. campaign in Syria has been waged by warplanes flying out of Qatar and other locations in the Middle East.