Okay, as is typical, this turned out longer than I had intended, but here it goes. Reid, you said this:
Now, it’s very much a liberal impulse (and a good and generous impulse) to support a people that exists as a minority spread across multiple states, so I’m certainly not going to knock that. But the YPG (which is the primary militia of Syrian Kurds, which plays a leading role in the SDF) are certainly not angels, and they shouldn’t be seen as occupying some unique moral high ground in the region. Some right-leaning analysts who have lost all sense of perspective in recent days and are trying hard to justify Trump’s decision by comparing the YPG to ISIS. But of course that’s gross hyperbole. Nonetheless, it is the case that the YPG has strong ties (even shared personnel, various types of cooperation) with the PKK, a Turkish-Kurd separatist organization based in Turkey that the Turkish government sees as a threat to the country’s integrity (i.e., a Kurdish state would mean the partial dissolution of Turkey) and that carries out terrorist attacks in Turkey. For analysts who follow this stuff closely, there’s a real difference between how the Syrian Kurds and the Iraqi Kurds are perceived. From what I gather, the Iraqi Kurds have actually been the one’s doing the very admirable work of establishing institutions and develop a proto-state, while the Syrian Kurds have been involved in terrorism and other unsavory things, including their association and cooperation with the PKK. In other words, the Syrian Kurds aren't "good guys," even if they aren't Al-Qaida (although, of course, from Turkey's perspective, they totally are).
Now the other thing: US support for the YPG undermines Turkey in the region. In the summer of 2014 when ISIS conquered all that territory in Iraq and Syria, the US stated it was its explicit policy that the integrity of the states in the region not be violated, and all maintain the same borders that they’ve had. There are a lot of reasons why the US would maintain the policy, but that was in part a concession to Turkey, because it doesn’t want a Kurdish state. Turkey has legitimate security concerns about the Turkish Kurds, and ever since the US and the YPG have collaborated, Turkey has always reluctantly accepted US support for the YPG — although they haven’t always “accepted” it at all, and at times its been a real point of friction between the US and Turkey in recent years.
Now here’s another thing: for very good reasons, the US has become allergic to using US military “boots on the ground” to accomplish its goals in the region, and so to maintain a “tight footprint,” it enlisted the help of the Syrian Kurds to fight against ISIS and has provided air support (and, of course, “military advisors”). Now something to know about that: it’s no secret that Turkey was threatened US support for the YPG. The US may have enticed the YPG with promises of an independent Kurdish state in return for their support in the fight against ISIS, but they would have been fools to believe that the US was going to go through with it. Turkey is too important a country to to the US for the US to turn its back on it.
And so about Turkey: Turkey has been a key part of US foreign policy in the region for decades. Turkey was the first Muslim majority country to recognize Israel, and Israel and Turkey cooperated frequently until Erdogan came into power, when the relationship became more fraught. During the Cold War and starting with the Since the Camp David Accords in 1978, when Egypt effectively left the Soviet sphere of influence and joined the US, Israel, Egypt and Turkey together formed an axis that was the core of the US’ Middle East policy.
Now that started to fall apart in the 2000s, in a kind of interesting way that I’ll just describe quickly. You may already know this, but the notion of a “deep state” comes from Egypt and Turkey: Turkey was effectively run by the country’s military establishment, which was secular, pro-US, pro-NATO and pro-integration of Turkey into Europe. Erdogan came to power as a populist who promised to wrestle power from the military establishment and make the government more responsive to the wishes of its people rather than the needs of the country as defined by its military/ He did a 180 on their tack when he came to power: he's made the country more religious, and turned away from NATO and Europe and instead has tried to direct the country outwardly towards the Middle East (a policy sometimes called “Neo-Ottomanism”: effectively the idea is that Turkey is a country on two continents, and throughout its history it’s seen itself as either European or Middle Eastern. Erdogan has lead the country to focus on expanding its influence in the Middle East, rather than trying to integrate more into Europe.)
Another thing to know about Turkey: Turkey has also been an historical rival of Russia. Currently, like Israel, and like Iran and the Syrian government, Turkey has to treat Russia as a power broker in the region. But it’d prefer not to do that as it expands into Syria and into Iraq (and yes, Turkey has for a long time already had a presence in both Syria and Iraq). So, given the historic hostility between Turkey and Russia, and the long-standing relationship between the US and Turkey, the two countries make, in some respects, a natural team.
And that leads us to the conclusion. People might complain about how bad the process was, how more diplomacy could’ve been involved, how the Trump admin could’ve done more to discuss with allies in the region before announcing the decision, and could have at least heard them out.* But it’s really not a ridiculous thing to support Turkey.
The media wants to moralize it and paint it as some kind of example of Trump getting chummy with an authoritarian leader (akin to his relationship with Putin or Duterte or Orban or Netanyahu -- hence the whole "rejection of the liberal international order" narrative). There’s of course a risk to assigning too much strategic thought to Trump personally, but there are actually still people with heads on their shoulders in his admin helping to guide foreign policy decisions, and siding with Turkey isn’t a terrible idea. At the moment, it’s difficult to square with the admin’s Israel policy, due to the hostilities between Turkey and Israel (hostilities to a certain extent manufactured by Turkey to increase Erdogan's credibility both domestically and internationally). But despite what you hear about the “Cold War” between Saudi Arabia and Iran, the big players in the region are actually Israel, Turkey and Iran. (Turkey and the Gulf states compete in the region over cultural trans-national Sunni leadership but AFAIK Turkey is far ahead in terms of winning hearts and minds.) So it makes some strategic sense to side with Turkey, especially as Turkey expands its influence in the region and increasingly finds itself at odds with Iran.
The Trump admin may have thrown the Kurds under the bus, but unfortunately it was only a matter of time: as I hinted at earlier, the Obama administration knew when it formed its alliance with the YPG that no US admin would ever side with the Kurds and drive a wedge between the US and Turkey. The outcome was plain to see to everyone from the beginning (and, in fact, if you look back you'll find no shortage of people who've been saying it).
*(As an aside, I see a lot of leftist anti-establishment people making these process arguments, which is funny because they usually criticize establishment liberal folk for putting too much emphasis on process. Is it a surprise that that’s the argument their making? They’ve been arguing for a significantly rolled back US military, and this is what that would look like. They got what they wanted — and that means pulling support for minorities like the Kurds. But I leave that as an aside.)