From Moon of Alabama:

The U.S. move comes at the right time for Syria. The Russian, Turkish, Iranian and Syrian agreement of Astana set up a de-escalation zone in Idleb governorate but committed the parties to continue the fight against al-Qaeda. The agreement was in imminent danger of breaking down as Turkey protested against the current Syrian operation against al-Qaeda in east-Idleb. Turkey cooperates with al-Qaeda to keep its options open for a take-over of some Syrian land. It is also concerned about the north-western Kurdish enclave of Afrin which is protected by Russian forces.

But the U.S.move in the east constitutes a greater threat to Turkey than tiny Afrin. The east is more important to Turkey than Idelb in the west. The whole eastern half of Turkey is now endangered by a Kurdish force at its underbelly. The U.S. move increases Turkey’s incentive to keep the Astana agreement about Idleb intact and to re-unite with Syria, Russia and Iran against the U.S.-Kurdish alliance…

Internally the area consists of a Kurdish core but has nearly as many Arab inhabitants as Kurds. The Kurds are not united, there are many who are against the PKK/YPG and support the Syrian government. Probably half of the Arabs in the area were earlier Islamic State fighters and the other half favors the rule by Damascus. What all Arabs there have in common is hatred for their new Kurdish overlords. This all is fertile ground for an insurgency against the U.S. occupation and its Kurdish YPG proxy forces. It will need only little inducement and support from Damascus, Ankara or elsewhere to draw the U.S. presence into a chaotic fight for survival.

Turkey’s wannabe Sultan Erdogan has long tried to play Russia against the U.S. and vice versa. He ordered Russian air defense systems which will enable him to withstand a U.S. air attack. At the same time he allowed U.S. ships to pass the Bosporus Straits into the Black Sea and to threaten Russia in Crimea even when the Montreux Convention would have allowed him to restrict their passages. The U.S. now leaves him no choice. Russia is the one force that can help him to handle the new threat.

The NATO bigwigs in Brussels must be nervous. Turkey has the second biggest army within NATO. It controls the passage to the Black Sea and with Incirlik the most important NATO airbase in the south-eastern realm. All these give Turkey leverage that it can use when Russia offers it a decent alternative to NATO membership.

From the comments:

US actions in Syria are dictated by a higher power. Israel.

Anything that weakens its neighbours strengthens Israel. In the meanwhile, the
idea seems to be that that border guard will impede passage of Iranians to Syria from Iraq on their way to Israel. The presence of American troops, once in the open makes the area sort of a no man´s land for Syria’s friends and Syria itself. The US can always claim that in attacking the SDF, Syria or whomever would be attacking US Allies which the US is “bound to defend”…

McMaster’s Ph.D. amounted to browsing minutes of meetings of the US top political brass during Vietnam War and then trying to “replay” it. It is not a real academic work…

So the US has recently made its presence in Afghanistan costlier and less sustainable by pissing off Pakistan. Now it locks itself into Kurdistan. Good gig for the aerospace part of the MIC, pity the ground forces…

4000 US boots on the ground in 14 bases, none of which could hold off a sustained assault by rocket fire, MLRS. Air cover for these bases is hourly deconfliction arrangements with Russia. Resupply is by air.

This is like 14 Khe Sahns…

So far, the moderates, the AQ, al Nusra and ISIS have all been humbled by Russian strategy, tactics, munitions and logistical support of Assad and his allies…

This should be over by July 2018 if done right. It’s an election year. The message of a defeat in Syria and bloody exit won’t play well for the neocons…

The USA want Erdogan to beg for his survival as a president…

[R]egarding possible supply routes into SDF territory, the US is absolutely free to land stuff in Basra and haul it towards the Iraqi-Syrian border. Have a look at who controls Iraq’s most important deepwater port in Basra (it’s interesting in its own right, even without the Syria context). Company called NAWAH? Fun fact, both founders/CEOs are heavyweights. One of them is ex-Pentagon and b-o-t-h are “Council on Foreign Relations”…

McMaster was a poor choice for US national security advisor. All he knows is army tactics and American history. He has not been educated in, and has no experience in, foreign affairs…

Here is the US + allies deployment in Syria, and here are Iran’s unimpeded land corridors to the Med…

Imperial planners are abandoning the little caution they’ve employed as their desperation increases. The times are more dangerous than 1962. But unlike then, we now know who the true enemy actually is and where they reside…

[A] lot of the US stupidity is actually the stupidity of Israel, which can send in its suicide fighter (the US) into stupid situations regardless of win or lose, but which also is stuck in its own trap in the Middle East and cannot think of a way out of its existential doom…

Trump is giving the deep state whatever hair brained schemes it wants to pursue secure in the knowledge that these stupid moves will in the long run destroy their hold on the world…

Erdogan is a two-tongued snake, but even he can see (certainly after the recent US-backed coup attempt against his own government) the writings on the wall. The next country in line ready to be cut into pieces isn’t Iran…

If Erdo knows what’s good for him, he’ll soon announce Turkey’s withdrawal from NATO and become firm in alliance with states that respect international law…

Turkey may have US by the balls by staying within NATO.

Turkey Notifies NATO Of Imminent Massive Invasion Of Syria To Fight Kurds