Should civil war break out as Americans depart, Iran would move to fill the gap with weaponry and perhaps volunteers to assist their Shia brethren in keeping Iraq in friendly hands. A Sunni-Shia war in Iraq, with Iran aiding one side and Arab nations the other, becomes a real possibility.
I agree with Pat. Things are looking dire. Our controlled chaos exit strategy may collapse into full chaos (particularly given any action relative to Iran's nuclear program).