The ISIS army has a very real possibility of turning Iraq into the next Revolutionary Islamic State in the mold of Iran. This would pose a threat to the US and its allies on economic and security interests.
Its my opinion that the US should have settled for a solid 20-30 year occupation like we had in Germany and Japan to prevent this sort of bullshit from happening, but the american people apparently were desperate for the immediate gratification of pulling the troops out ASAP.