While the Iraqi army has completed failed on many occasions recently, keep in mind they've been fighting these guys for months. It seems what we call Iraq will probably be reduced by 2/3rds unless they can get the Kurds & Turkey to help. Maliki probably can't, so his only realistic is hope he has is to get the US & Iran to help him. Iran has an interest in protecting Shi'ite shrines & the US needs oil prices stable.
While its certainly not in the US' interest to surrender territory to the guys kicked out of Al Qaeda for being too extreme, the territory they have isn't as important as the region currently under control by the Iraqi government (with the exception of the refinery currently under dispute). No way the US sends in conventional troops, considering they have little chance of success convincing the Sunni's they previously fought with to fight insurgents during the war to cooperate with Maliki's forces. The Sunni's recently "conquered" by ISIS didn't fight them too hard and even though they don't want a religious state, they think ISIS will treat them better than Maliki.
In short, Iraq's ****** & turned out to be a very bad & expensive investment. We'll probably drop some bombs in order to protect the current area controlled by Iraq and keep the sweet, sweet crude flowing, but its gonna be up to Maliki to try and take back what he lost (if he wants another civil war).
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"The Rich are there to take all of the money & pay none of the taxes, the middle class is there to do all the work and pay all the taxes, and the poor are there to scare the crap out of the middle class." -George Carlin