The root cause of this crisis isn't Bush or Obama it's al-Maliki. It's his polices and actions that have lead to this disaster. If he not removed from office but the Iraq parliament, then there is nothing the US can do to stop this problem. The Iraq army didn't collapse because US pulled force out, it failed because al-Maliki promoted officers that had political ties to him vs merit. The Sunni tribe are siding with ISIS because al-Maliki forces in the area were corrupt and stealing for the local population.
And if the US engages now then we just keep the problem in power and ensue that his patrons in Iran continue to have strong influence over Iraq