Russia's Pulling Out Of Syria...Why Can't We?

With ISIS defeated in Syria, Russian President Putin announced this week that he was withdrawing the bulk of Russian military equipment and personnel. Russian assistance was requested by the Syrian government as ISIS and al-Qaeda appeared poised to take control of the country. Meanwhile, the US has just admitted it has more than four times the number of troops in Syria than it earlier claimed. And Defense Secretary Mattis said the US, though illegally in Syria, would not be leaving. Why?