If you go back to 2007/08, Gen Allens job in Iraq was to try to shift the Sunni to supporting the central Iraq govt and he was successful. That is why Obama gave him the job in Afghanistan to shift the Taliban.
And this is why Obama gave him this job in Iraq. The main job in Iraq will be to convince the Sunni to abandon their support for ISIS.
As for the rules of engagement, those are found in Counterinsurgency Manual used by both the Army and Marines. Gen Petraeus had the manual rewritten and he hired Col John Nagl to rewrite it. Nagl was the Rhodes Scholar of Counterinsurgency. Petraeus wrote his college thesis on counterinsurgency. Nobody had used the old manual since VietNam.
Thanks.