update 2: A troop withdrawal appears already underway after a Pentagon official said it would happen "quickly".

White House spokesperson Sarah Sanders has issued a formal statement on troop withdrawal from Syria: "We have started returning United States troops home as we transition to the next phase of this campaign."

Moments after President Trump confirmed reports of US pullout via Twitter saying "We have defeated ISIS in Syria," Pentagon officials said the president "ordered full US troop withdrawal from Syria," and that this will be "rapid" — apparently already beginning, per a Reuters breaking report: "All U.S. State Department personnel are being evacuated from Syria within 24 hours - official."

The full White House statement issued Wednesday late morning:

Sarah Sanders: “We have started returning United States troops home as we transition to the next phase of this campaign...” pic.twitter.com/1V00r4WUyP — Kaitlan Collins (@kaitlancollins) December 19, 2018

Meanwhile Israel's ambassador to the U.N. has vowed to continue combating Iran in Syria if US forces leave. Russia has alternately said "Syria will stabilize" should the thousands of American troops stationed there exit.

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update: It's official, within an hour after the first headline the president tweeted "We have defeated ISIS in Syria" and in reference to reports of a planned US troop withdrawal from Syria which unnamed officials say is to be initiated "immediately," he added that the terror group's defeat was "my only reason for being there".

We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency. — Donald J. Trump (@realDonaldTrump) December 19, 2018

Minutes after the WSJ first broke the story, The Washington Post confirmed the following through a defense official:

The official, who spoke on the condition of anonymity to discuss a decision that has not yet been announced, said the decision would include the entire force of more than 2,000 U.S. service members. It was made on Tuesday, the official said. President Trump has long promised to conclude the campaign against the Islamic State and has questioned the value of costly and dangerous military missions overseas.

***

The WSJ just reported a monumental and historic reversal in White House policy on Syria, revealing Wednesday morning the Pentagon is preparing to withdraw all forces from northeastern Syria "immediately":

In an abrupt reversal, the U.S. military is preparing to withdraw its forces from northeastern Syria, people familiar with the matter said Wednesday, a move that throws the American strategy in the Middle East into turmoil. U.S. officials began informing partners in northeastern Syria of their plans to begin immediately pulling American forces out of the region where they have been trying to wrap up the campaign against Islamic State, the people said.

US forces in Syria, via ABC News

The WSJ notes the complete 180 reversal in policy, which just days ago was reiterated by officials as an "indefinite" American presence in Syria in order to "counter Iran" while bolstering Kurdish and Arab SDF forces in the East (the YPG-dominated Syrian Democratic Forces), comes following a phone call last week between President Trump and Turkish President Recep Tayyip Erdogan.

For the past week Erdogan has threatened to launch a full-scale cross border assault on US-backed Kurdish forces in Syria, which Turkey has long considered an terrorist extension of the outlawed PKK. This would potentially bring American troops and advisers under fire, who've found themselves in the awkward position since entering Syria of training Syrian Kurdish militias on the one hand, and coordinating broadly with a NATO ally on the other.

Perhaps Trump finally took full stock of the fact that the prior planned "indefinite" presence of some 4000 American troops was recipe for a quagmire sure to be Washington's next Afghan or Iraq style "endless war"?

As one recent intelligence study put it: "The prospect of US being militarily involved in Syria, caught in middle of one of most complex conflicts in recent memory, with shifting objectives & ambiguous endgame, has been met with congressional indifference and public apathy."

Will America finally exit Syria? Is Trump belatedly making good on his campaign promises?