Creative Speculation

They went from next to nothing to holding sizable territory within 2013. Watch this: http://www.pbs.org/wgbh/pages/frontline/rise-of-isis/

The program faults the Obama administration for its rise, but due to inaction: first in not stopping al Malaki from turning against the Sunni population, purges and violence that included a major massacre of hundreds of Sunnis; and by not arming more moderate rebel groups in Syria.

Part of what makes ISIS so dangerous is they have highly trained military men from Saddam's Baath party. It makes them capable of far more than what we think of as terrorist attacks. The Iraqi army is far less capable militarily than ISIS. Assad's army is a different story.

The source is obviously pro-Shia and pissed off that the Obama administration insisted Malaki step down before it would intervene in Iraq. The US does not want to make it a Shia war against Sunnis (which is how and why ISIS emerged in the first place).