The US-Turkey relationship is one of mutual need for each other. The US needs Turkey because of its formidable military and being a NATO fortress on the edge of the Middle East, but Turkey needs the US even more simply because of the fact that if you piss off the US too much
But I
don't think that the US needs Turkey more than Turkey needs the US, no. Rather, I think that the US needs Turkey more than it needs SDF, which is a completely different equation.
SDF is simply not important or valuable enough for the US to completely sacrifice its relationship with Turkey and engage in some kind of cold war with it over the issue - the SDF doesn't have enough to offer the Americans that could replace the value of Turkey. Thus, the US would obviously prefer if it had both Turkey and SDF to play off against each other, but if it had to chose between SDF and Turkey, it would most likely chose Turkey, even if the Turkish behavior was not all to America's liking. Of course, the same could be said about Russia as well to an extent, however Russia has less to lose if it pisses off Turkey, because the Russian-Turkish overall relationship is not as good and never has been, and their interests are still somewhat conflicting in many areas of the world, while Russia's relationship with the Kurds and specifically PKK is historically a lot stronger.
Less importantly but still worth mentioning is that it would not be entirely easy for the US to even come up with a reason to generate enough hatred for Turkey on such short notice if Turkey invaded the SDF territory with the permission from Damascus government. There's nothing at all illegal about that move, and it is only questionable at all in so far as it disrupts the fight against ISIS (what about after ISIS is defeated though, what then?), so how can you even justify any kind of meaningful response to it? Over time some excuses can be made to slap Turkey with sanctions if that is really the desired course of action, but by that point it might be too little and too late - the SDF would have already been dealt with blows from which they can never recover. The Russian/Iranian response can be much swifter, and due to the support of official Damascus, they can stand on much firmer ground.
ISIS defends large portion of Deir Ez Zor province because it is technically a supply line.
If they centered their defense around Raqqa, then my assumption would be that they would retreat from Iraq much sooner to bolster their defense in Raqqa, therefore negating the value that Deir Ez Zor has as a supply route to Iraq. Basically I was expecting ISIS downfall to look more like Nazi Germany's, which centered its defense around Berlin and concentrated forces in such a way as to make sure that its opponents do not get to Berlin unless the rest of the country would have already fallen, it was clear that the Germans did not plan to continue the war if Berlin fell. So indeed after Berlin fell, there was not much left of Nazi Germany - there were only small pockets in Czechia, Denmark, Norway, and the mountains of Austria. With ISIS we are seeing something different. They seem to be planning to continue the war after the loss of Raqqa, and they are centering their defense elsewhere. I guess all that oil in Deir Ez Zor must be making them mad profits or something, so they find it preferable to make Deir Ez Zor into their last stand instead.
We'll all probably get cancer if you keep this up.
Igor is a gift that keeps on giving. And of course, no source for his other claim either.