The rise of ISIS is pretty simple, the power vacuum created by the destruction of the Ba'athist Iraqi state and the subsequent withdrawal of US troops was fille

The rise of ISIS is pretty simple, the power vacuum created by the destruction of the Ba'athist Iraqi state and the subsequent withdrawal of US troops was filled in Shi'a areas by Shiite militias and (largely Shiite) Iraqi government forces, in Kurdish areas by the Peshmerga, and in the Sunni areas by the Sunni militants who had been fighting the US troops. Lots of groups fought the US of course, a number were wiped out to varying degrees. Because the US held an overwhelming advantage in conventional firepower, the only groups with any lasting "success" were the ones that embraced terrorism as their core of their operations. When the Americans left the strongest of these groups -- ISI -- consolidated power and the destabilization in Syria created another power vacuum which ISIS flowed into.