In a major shift in United States military policy in Syria, the White House said on Sunday that President Trump had given his endorsement for a Turkish military operation that would sweep away American-backed Kurdish forces near the border in Syria.
Turkey considers the Kurdish forces to be a terrorist insurgency, and has long sought to end American support for the group. But the Kurdish fighters, which are part of the Syrian Democratic Forces, or S.D.F., have been the United States’ most reliable partner in fighting the Islamic State in a strategic corner of northern Syria.
Now, Mr. Trump’s decision goes against the recommendations of top officials in the Pentagon and the State Department who have sought to keep a small troop presence in northeast Syria to continue operations against the Islamic State, or ISIS, and to act as a critical counterweight to Iran and Russia.
Administration officials said that Mr. Trump spoke directly with President Recep Tayyip Erdogan of Turkey on the issue on Sunday. And the officials indicated that the 100 to 150 United States military personnel deployed to that area would be pulled back in advance of any Turkish operation but that they would not be completely withdrawn from Syria.
“Allowing Turkey to move into northern Syria is one of the most destabilizing moves we can do in the Middle East,” Representative Ruben Gallego, an Arizona Democrat and former Marine who served in the Iraq war, said on Twitter on Sunday night. “The Kurds will never trust America again. They will look for new alliances or independence to protect themselves.”
Officials described a military and political tension as the American military is pulled between two important allies in the civil war in Syria. Turkey is a major NATO ally, but the Kurdish S.D.F. forces have been a partner in the fight against ISIS.
“We are not going to support the Turks and we are not going to support the S.D.F.,” the official said. “If they go to combat, we’re going to stay out of it.”
Read it all. 
Here’s an explainer.  Excerpt:
The prospect of a Turkish military push into northern Syria has caused deep fear in Kurdish areas there, as well as a burning sense that the Kurds have been betrayed by the United States after years of partnership on the battlefield.
The Syrian Democratic Forces  — a loose coalition of militias that is led by Syrian Kurdish fighters and came together expressly to fight ISIS with American backing, training and air support — accused the United States on Monday of failing to fulfill its obligations, paving the way for Turkey to invade.
The S.D.F. also warned that a Turkish incursion could undo the gains made against the Islamic State.
I will update this post as I read more about this situation, but my first impressions are these:
No question, the United States has sold out an ally. Morally it’s a disgrace. But I’m not at all sure that Trump has done the wrong thing here. In fact, though I’m open to having my mind changed (let’s hear your arguments!), I think he might have done the right thing in an ugly way.
Of course the usual Washington foreign policy claque is howling at Trump for doing this. One prominent voice is taking his side:
I stand with @realDonaldTrump  today as he once again fulfills his promises to stop our endless wars and have a true America First foreign policy.
— Senator Rand Paul (@RandPaul) October 7, 2019 
Why should we stay involved in Syria? I’m not asking rhetorically — I’d really like to know. As the Times story points out, the only allies we have are the Kurds. The Turks — arch-enemies of the Kurds — have an obvious interest there, and they have had to bear an enormous burden of caring for war refugees. They’re understandably tired of it.
The Russians have an obvious interest too. The Assad regime is their longtime ally, and the Russians have a Mediterranean base on Syrian territory. Iran has also long been an ally of the Assad government, seeing Assad-run Syria as necessary to balance US and Saudi influence in the region.
What is the compelling interest for the United States to remain militarily engaged in Syria?
For years, ISIS has watched as America’s reliance on the YPG/PKK [Kurds] has enervated the second-largest army in NATO. It has cleverly preyed upon this tension by waging terror attacks in Turkey it declined to claim credit for, knowing full well that Erdogan and his government might blame them on the PKK, which it did on more than one occasion.
Turkey, meanwhile, had maintained a see-no-evil policy (or worse) with respect to ISIS as its sovereign territory became a staging ground for exported jihad, not to mention a fallback base of operations for the many agents of ISIS’s Amniyat, or intelligence service, who were dispatched out of Syria and into Western Europe.
“Trump considers the Middle East a place of sand, death, and oil that America isn’t stealing but should.”
In other words ISIS leader Abu Bakr al-Baghdadi (yes, he’s still alive) exploited the Kurdish Question before and will exploit again now that all-out war between Turkey and Syria’s Kurds is a foregone conclusion.
And so long as the inevitable carnage stays in the neighborhood, we can be reasonably sure that Donald Trump won’t care or will treat this as just the latest manifestation of a regional pathology, and not an unforced error of lousy U.S. policy-planning.
If Trump’s move will likely allow the resurgence of ISIS, then do we not have a reason to remain in the region? On the other hand, it seems like there is no exit strategy — that we will stay pinned down in Syria forever. Are we prepared to do that?
In related news:
— Adam Tooze (@adam_tooze) October 6, 2019 
What about the Christians in the region? A European friend texted this morning with great alarm, saying that this move guarantees the destruction of what remains of the historic Christian communities there. America — pious America, led by Evangelicalism’s Christian Warrior President  — pretty clearly does not give a damn about those people.
To refresh your memory: I generally favor exiting from America’s foreign wars. I am non-interventionist in my foreign policy orientation. The fact that so many interventionists are screaming bloody murder at Trump this morning makes me instinctively side with him. Maybe Trump has done the right thing here, though in an ugly way.
Or maybe he has done the wrong thing. Maybe the cost of withdrawing from Syria in this particular case is not worth it. Maybe the harm of the betrayal of a US regional ally, and the exposure this now gives to the defenseless Christians of the region, outweigh non-interventionist principles in this particular case?
What do you think? Not that anything we believe is going to change matters. That die has been cast, it appears. (Though this move has upset some leading GOP senators — so far, I’ve seen anger from Graham, Rubio, and Sasse — and that may well make them less likely to stand with Trump in a Senate impeachment trial.)
Whatever you think about Trump’s move on Syria, let us recall that we — the United States of America — lit the fuse on all this with our unjust war on Iraq.
UPDATE: From our Big Cheese editor, this truth:
Wacky tweets and a chaotic policymaking process aside, it says a lot about the state of DC that Trump may have marginally increased his chances of being removed from office by attempting to end a war Congress never authorized in the first place.
— Jim Antle (@jimantle) October 7, 2019 
I'm not against getting out of Syria. I'm against using the opportunity to encourage a NATO ally to basically annex part of it and commit ethnic cleansing.
— Michael Brendan Dougherty (@michaelbd) October 7, 2019 
UPDATE.3: John Zmirak has been a huge Trump supporter. He’s bailing on this issue.  Excerpts:
Trump seems about to throw away the American coalition’s victory over ISIS. The Syrian Democratic Forces who did the bulk of the fighting captured many thousands of ISIS fighters. They hold them and their families, an estimated 60,000 people, near Iraq border.
If Trump lets Turkey and its al Qaeda jihadist allies invade the Kurdish heartland, the SDF won’t waste its soldiers on guard duty. It has announced it will release the ISIS fighters. Overnight, ISIS is back. It will likely stream into nearby Iraq, and prove the last straw in collapsing that country’s already fragile government. Christians and Yezidis will once again be killed, raped and sex trafficked. Iraq will likely collapse again into civil war. That will leave the only power in the region capable of picking up the pieces: Iran.
We are pulling out our troops, so Turkey can repeat the butchery it committed in Afrin, Syria in January. But if Turkey does again what it already did once and has promised to do again? We’ll slap it with … economic sanctions. Is that likely to impress the dictator who wants to re-build the Ottoman Empire? A man who has clapped tens of thousands of journalists in jail?
Look, I’m a skeptic of U.S. intervention. I was one of the few conservatives to oppose the Iraq war back in 2002. But U.S. soldiers aren’t fighting in Syria now, and Trump’s not bringing them home. He’s just moving them around from where they are useful to where they’ll be useless.
Trump may very well alienate key allies by this move, at a time when he needs them most. The Christian right has spoken up more and more about the fate of Syrian Christians. At last we make some atonement for our total neglect of Christians in Iraq, 75 percent of whom Islamists killed or made refugees while U.S. troops looked on.
I don’t think the Christian Right (meaning, pretty much, white Evangelicals) is going to seriously back away from Trump over a bunch of non-Evangelical Middle Eastern Christians. I would like to be wrong about that.