Is exactly what they have been seeking all along, and we have now no possible way to deny them. They seek the south of Iraq as their own satellite state. If it remains attached to the dead husk of Iraq, they don't care- as long as the Sunnis cause no trouble. The Kurds will be more than pleased with the arrangement. The US has no say in this once we leave the area.
Its not Iran that you should be worried about. The Shi'a dominated government of Iraq is quite happy with this idea. Iran is quite happy with this idea. Who you should worry about is Saudi Arabia- who will not stand for a strong greater Shi'a alliance. Who will not stand for Sunni Arabs playing the role of oppressed politically powerless minority. Watch what they do with their billions of wealth. As they have already been doing- they will fund and foster a fundamentalist Sunni rebellion, and Crackdowns by the Shi'a government will only fuel the rebellion.
Blood in the streets within this decade, and a new totalitarian state, this time friendly to Iran as the result. A proxy war in the central and north of Iraq between the wealthy Sunni arab states and Iran.
Yeti