It is within the buffer zone that trump gave Turkey. They have de facto control over the area. trump later begged Erdogan for something, anything, in return, but the deal had already been made.
Assad had lost the territory years before. Anyway, his main objective is to control what is within the borders, not the borders itself.
Turkey is a NATO nation, so we are allied with Turkey. So much the same reason trump had so many allies, not because of who he is, but the country he led. Did you see how little everyone thought of trump? If we do that to the world again, they may just quit us.
