The Kurds operating in NE Syria are opportunists of the highest order. They displaced most of the Syrian Arab population (in 2010 Kurds made up 7% of the population - now it is +70%) and have been actively supporting their sister terrorist organisation - the PKK - in south eastern Turkey. They have actively courting the brutal al-Assad regime and been butt-snorkling the Russians.
Turkey has very real security concerns along their southern border - which the US (principally CENTCOM) have, at best, paid lip service to. Turkey's greatest fear has been the establishment of a contiguous Kurdish Canton from NW Iran, across northern Iraq and then along Turkey's border with Syria - and ultimately, to the Mediterranean. This would have established "Kurdistan' as a
de facto state. The only thing that would unite Iran, Iran and Turkey in any sense is their implacable opposition to a Kurdish state. KRG was about the limit in Northern Iraq, but their referendum two years ago out paid to their uppity behaviour.
Of course, it's not the first time the US have abandoned the Kurds in the region. The Treaty of Sevres 1919 proposed the establishment of a Kurdistan under a US Mandate. Post war Isolationism (like now) killed off that bright idea.
Now, how are the US going to recover the thousands of tons of weapons they issued the YPG* over the last four years - as they promised Turkey they would do?
* short for Yanks Provide Guns.
Here's an a recent article in the independent Al Monitor about the issue of disarming the YPG. Metin Gurcan (retired Turkish SF Colonel) can be a bit sensationalist on occasions but this is a balanced article.
https://www.al-monitor.com/pulse/ori...isarm-ypg.html