It would be an incredibly tricky thing to accomplish, but if the Iraqi government made a REAL genuine turn from sectarian government and the Sunnis believed it, you could have an organic repeat of what happened in 2006/2007.
In 2006, Iraq fell into an intense civil war. McCain and others credit the surge with ending it. More thoughtful people note that even before the surge, the civil war was changing as Sunni leaders started to throw out the AQ aligned terrorists. I have heard 2 reasons given - one that the Sunnis feared for what would happen to their own families and realized the cost of hosting the terrorists and that the US paid some leaders to make that decision. (If the latter, it might be one of the few times I think bribery could be justified.)
What is even more complicated this time is that ISIL are mostly ethnic Iraqis. They have nowhere to go - and there are some that note that they become worse in Iraq as they lose ground in Syria. They are extremely radicalized and extremely violent. It is hard to see either where they go or how they could become part of a nonsectarian Iraq. The question would be how many of these people are hard core terrorists and how many could return to a non sectarian Iraq.
It does not make sense to bomb population centers - as that will just kill innocent people and probably increase the ranks of ISIL. Probably the only exception is that if the ISIL can be attacked directly when they are in convoys from one town to another town they intend to attack, if the US had 100% proof, this might be the only justifiable if done at the request of a NON SECTARIAN (ie not Al Maliki ) government. It would be preventing a massacre. (If the government is not non sectarian, the problem is that most Sunnis could still see it as the US attacking "them" for the Shiites.)
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