I still don't understand why the US won't fight IS together with Russia and Assad. I though it was well known by now that the FSA and IS are difficult to take a

I still don't understand why the US won't fight IS together with Russia and Assad. I though it was well known by now that the FSA and IS are difficult to take apart, they are basically mercenaries fighting for whoever has money. When IS got oil wells (and the means to make money off of them by secretly selling to Erdogan) the FSA, at least partly, became IS. Assad has been saying since the beginning that the FSA are animals, and some of them were barbecuing heads at some point.