I don't see how anyone can dispute a civil war if Americans are not there to hold things down anymore.
Even with Americans there, Sunnis are blowing car bombs up in markets, mosques and funerals, and Shiites are going around at check points and bus stations checking for Sunni names such as Omar, taking them away and using electric drills on their bones and skulls until they die. Electric drills is a specialty of some in Iraq. Up to 1000 a month are killed this way, if not more. Tens of thousands are witnesses that it's Sunnis and Shias killing each other - the Badr Brigade, al Sadr's militia, Al Qaeda in Iraq, Saddam's ex security officers.
Are we going to assume that should the Americans leave abruptly, the Shias and Sunnis are going to say, "hey buddy, the reason we're attackign *each other* is to make the Amis leave?" How does that work?
I am not justifying the war. In the light that there is no WMD, there is much less justification for the war and America has been humiliated as a result. and if Iraq fails because the people just want to kill each other, fomented by its neighbouring countries, then it would be doubly so.