So Assad, who could not quash a civilian uprising in his own nearby cities
He was winning on all fronts before the west intervened, had they stayed out of it he would have had the rebels beat before ISIS became an issue.
could have swept aside a fanatical terrorist group littered with seasoned fighters and equipment?
Yes, notice how he hasn't been using his air force properly against the rebels out of fear of US reprisal? He wouldn't have that problem with ISIS and his military could have gone full force (plus he could have asked Russia for help if he needed it).