This has a partially factual answer, but because it relates to the Iraq war I will start it here. Move as necessary.
Since the Iraqi government shows little sign they are capable of resolving the differences between the three principals in the sectarian violence, it is likely the Iraq civil war will escalate when we finally leave, whenever that is.
So, historically, how have previous civil wars in other countries finally concluded? I can think of only four possible outcomes:
[li]The strong violently overwhelm the weak, and the state is ruled by the majority.[/li][li]The country is partitioned, and the belligerents withdraw to their respective corners.[/li][li]The civil war continues ad infinitum in a balance of terror.[/li][li]The parties resolve their differences and live in peace.[/li][/ol] Are there any other outcomes from history we can reference? What is the likelihood for each of these in Iraq?
For myself, I believe #1 is the most likely outcome. I have no cite, but IMHO historically it is the most common outcome for civil wars. Even if we kill or capture all of the Al Qaeda in Iraq, the bitterness between the Sunni and Shiites is not going to go away simply through good intentions and diplomacy. The Shiites are going to kick ass until the Sunni prostrate themselves, and that may not happen until the last man is killed. Bloodbath.
#2 is the next most likely, but would have to be imposed from outside the nation, because the Shiites know they have the power to take all the marbles. Probably way behind #1, because nations like Turkey and Russia oppose it.
#3 is a possibility, but a remote one, because it would require continuing support for both sides from outside, and even Iran and Saudi Arabia have limits to their patience. Unless, of course, the US stays in between the combatants…forever.
And #4 is just plain unrealistic, given the animosity between the Sunni and the Shiites, and the Kurds have had a taste of independence they will be loathe to relinquish… Extremely unlikely.