From Moon of Alabama:

Trump promised to stop the delivery of weapons to the YPG forces in east-Syria. But the White House was evasive on the issue and the U.S. military Central Command has acted contrary to that promise…

The realist-hawks and NATO proponents are on Turkey’s side while the neoconservative “liberal” forces are on the Kurdish side…

It is not only the Incirlik air-base which is irreplaceable for NATO’s southern command. Turkey also controls the access to the Black Sea and has thereby a say over potential NATO operations against southern Russia and Crimea…

Unless the U.S. changes course and stops fooling around with the Kurds Turkey will continue to disentangle itself from the old alliance.

From the comments:

Turkey was lost by the CIA/State Dept of Obama. They undermined the Air Force of Turkey as part of the plot to assassinate Erdogan, not just remove him. He was to be shot out of the sky. Russian Intel services SVR and GRU saved him and protect him still, as well as have worked with his military to help rebuild morale, discipline and fealty.

From Ghassan Kadi at The Saker:

America had been looking for half an excuse to invade Syria for a very long time, and knowing that it wasn’t able to have a full-on presence that would allow it to carpet bomb the whole nation, it used the Kurdish excuse and the false pretext of creating a “security zone” in order to justify its presence on Syrian soil; against Syria’s wish.

But to do this, America needed allies on the ground, and instead of working together with its natural partner and NATO member Turkey (which happens to be a regional superpower) on common denominators, America’s inability to negotiate and give and take, even with its most ardent allies, repelled Turkey and America had to resort to an alliance with the YPG Kurds…

Arguably, it can be said that the way the YPG went to bed with America has led to its abandonment by the Syrian Government…

Erdogan had told his American “allies” time and time again that they cannot be strategic allies of Turkey if they want to endorse any form of a formal Kurdish entity…

Erdogan knows well that only he can engage in fighting American cohorts in Syria, and he is doing it with or without any need for support, not even tacit support, from either Russia, Iran or Syria…

[H]e regards America as the nation that hosts and protects his political enemy and rival Gulen; who ironically still enjoys a huge level of support within Turkey, despite all the clampdowns on his supporters after the July 2016 coup attempt.