the reason isis is so powerful is that they have the support of the masses...
The main recent results in Iraq are due to a large scale Sunni uprising against the central government. As the so-called
Al-Quaeda-in-Iraq, ISIS is a bit player, and relative side-actor that's well versed in self-promoting propaganda that goes well beyond its own practical ability, capabilities, and accomplishments.
Much of the media and unfortunately some naïve foreign affairs notables have been trolled into propping us this ISIS as the steamrolling juggernaut that it is not. It is a bit-player following and marginally using the swaying tides of more significant, numerous, and influential actors/groups in the population that have long been marginalised by the Shia dominated central government... Much of this goes back to the idiocy of the US and Britain with their criminally and incompetent warfare into Iraq, followed by the occupational disbanding/ostracising of the massive and influential Sunni segments from the population out of the military and government.
Iraq, as stitched by Britain, was always going to have some level of ethnic tensions and strife. In rate and veracity, the bloody Yanks exacerbated this to the extreme.