WASHINGTON (AP) — Against the advice of many in his own administration, President Donald Trump is pulling U.S. troops out of Syria. Could a withdrawal from Afghanistan be far behind?

Trump has said his instinct is to quit Afghanistan as a lost cause, but more recently he's suggested a willingness to stay in search of peace with the Taliban. However, the abruptness with which he turned the page on Syria raises questions about whether combat partners like Iraq and Afghanistan should feel confident that he will not pull the plug on them, too.

"If he's willing to walk away from Syria, I think we should be concerned about whether Afghanistan is next," Jennifer Cafarella, the director of intelligence planning at the Institute for the Study of War, said in an interview Wednesday.

The U.S. has been at war in Afghanistan for 17 years and still has about 15,000 troops there helping government troops combat the Taliban. The approximately 5,000 U.S. troops in Iraq are training and advising Iraqi security forces as they continue to fight Islamic State militants, a battle the U.S. entered in 2014 after IS swept into Iraq from Syria.

Before other officials confirmed the withdrawal decision, Trump tweeted, "We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency." The aspect of this that he did not address is whether the extremists or others will fill the security vacuum created by the U.S. withdrawal to regroup and pose a new threat.

The administration said it intends to continue combatting Islamic State extremists globally and could return to Syria if necessary. Still, critics launched a barrage of questions about the implications of Trump's decision, including whether it opens the door for Turkish forces to attack the Syrian Kurds who had partnered with the U.S.

Kori Schake, deputy director-general of the International Institute for Strategic Studies, wrote on the Atlantic.com website Wednesday that the Syria decision ought to unsettle every ally that relies on U.S. security assurances.

"The governments of Iraq and Afghanistan ought to be very, very worried," she wrote. "For if Syria can be so lightly written off, the fight arbitrarily declared won, what is the argument for continuing to assist Iraq — where ISIS is even more defeated? And if Trump has so little interest in stabilizing security and assisting governance in Syria, how can Afghanistan have confidence that he won't make the same decision about them, when the fight there is costlier and progress less evident?"

These and other questions about the Trump decision and its broader implications were on the minds of many in Congress. Sen. Jack Reed, a Rhode Island Democrat, denounced what he called a betrayal of the Syrian Kurds.

"Now the President seems content to forsake their trust and abandon them to a potentially bloody conflict with Turkey," Reed said. "This decision also significantly increases the security risks to our key regional partners in Israel, Iraq and Jordan."

Trump has argued for a Syria withdrawal since he was a presidential candidate in 2016, and he has repeated his view several times since taking office.

On Thursday, Trump defended his decision, saying on Twitter: "Getting out of Syria was no surprise. I've been campaigning on it for years, and six months ago, when I very publicly wanted to do it, I agreed to stay longer."

He added: "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?"

Still, the decision appeared to catch many in his administration by surprise; Pentagon officials offered no details on the timing or pace of the withdrawal, nor could they square it with numerous statements by Defense Secretary Jim Mattis about the importance of remaining in Syria to assure stability.

Sen. Lindsey Graham, a South Carolina Republican and typically a Trump supporter, said he was "blindsided" by the decision and called it "a disaster in the making." He said, "The biggest winners in this are ISIS and Iran."

Republican Sen. Marco Rubio of Florida said the withdrawal would be a "grave error with broader implications" beyond the fight against IS. He called it "one more example of how the United States is not a reliable partner."

Just last week, the U.S. special envoy to the anti-ISIS coalition, Brett McGurk, said U.S. troops would remain in Syria even after the Islamic State militants were driven from their strongholds.

"I think it's fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring," McGurk told reporters on Dec. 11. "Nobody is declaring a mission accomplished. Defeating a physical caliphate is one phase of a much longer-term campaign."

And two weeks ago, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said the U.S. still has a long way to go in training local Syrian forces to prevent a resurgence of IS and stabilize the country. He said it will take 35,000 to 40,000 local troops in northeastern Syria to maintain security over the long term, but only about 20 percent of them have been trained.

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Associated Press writer Lolita C. Baldor contributed to this report.