So 2 slightly different versions of the religion of peace have decided they want to kill each other? I'm struggling to see the downside to that one mate!
I don't think they have no other option than to return to the cities. The Iraqi police force or Army is incapable of keeping peace. Their guards are unable to check cargoes and the situation must be extremely lax. The problem if the US do have to return is for how long? If you have 2 resident populations the dominant Shia will impose their will and the less influential Sunnis will harbour grievences evermore.
Saddam was removed under the directions of militant Shias, we now know he was the only one to keep some sort of order. We have completely misread the situation in Iraq and we are paying for it.