SDF agreeing to Assad’s rule is a BIG WIN for Turkey.
They could go back home a winner if they want now. Assad will keep a tight rein on the Kurds.
Assad will see to a lot of refugee resettlement there.
Or they could turn to Idlib province.
No idea what kind of deal could be agreed on for Idlib.
I think Turkey wanted to take the land for keeps - same as Afrin and Idlib.
It those other areas they conducted massive ethnic/sectarian cleansing, to push out Kurds and Shi’ites, and pumped in a lot of Sunnis - especially the militant Wahabbi style.
They set up essentially Turkish Government in those locations - Turkish Post Offices, the whole nine yards. I believe that when the final lines settle out and the shooting stops, they plan to hold referendums on joining Turkey, and annex them permanently.
So the Syrian Government (Assad) and the Syrian Kurds are naturally on the same side against Turkish aggression - they just could not agree among themselves who the boss would be, and how things would work out. The Kurds were using their US backing to drive a harder bargain for autonomy from the Ba’athist Government.
They both knew that they would have to deal with Turkey and Erdogan after ISIS was cleared. That day, this new phase of the war, has come.