The Yanks are there but they are but not wanted by the lawful government of Syria. They are there illegally. The Russian's stated the shooting down of the Syrian warplane was an act of aggression. Whilst of course true, it would be more accurately described as an act of war... one of a long series committed by the US in Syria. They are trying to partition the country in order to protect the forces they favour who just happen to be terrorists, rebranded ISIS killers. The same ones they allowed to escape from Mosul and Raqqa to continue the fight against Assad.