"There are two sides fighting in Syria - Isis and Assad's Government forces "

The over simplification of the decade. What idiotic rag did you get that jewel from? Inside Syria there are a large number of factions representing the rebellion against Assad and his allies (Hezbollah, Iran and the Russians). Those factions are not only fighting against Assad they are also fighting against one another. ISIS took advantage of the latter conflict to carve themselves what they see as "their" Caliphate.