If the United States leaves, Iraq will descend into a full-scale civil war. The Iranians will come in on the side of the Shiites. The Syrians, Saudis and God knows who else will be tempted to come in on the side of the Sunnis. The Turks will be tempted to come in to take care of the Kurds. We might be looking at the Middle East version of World War I

i was opposed to the invasion from the beginning
however now we have to deal with 'facts on the ground' rather than about the justifications for the war
should we withdraw quickly and watch Iraq collapse or stay and commit to nation building
it isn't like Vietnam where there was virtually a government in waiting
should we commit to a long occupation since the insurgents know there are political forces in the US and Britain pushing hard for a quick withdrawal so the insurgents from their point of view have the psychological upperhand, a short sharp aggressive push and the occupation forces pull out and the puppet regime swiftly collapses.
The article warns not just of a civil war but the danger of others being drawn in and a wider conflagration.
The war was immoral but do we compound the error by leaving swiftly and letting the state descend into anarchy or do we add oil to the fire by continuing an illegal occupation, act as a recuiter for al-Qaeda and unite those who oppose the occupation.
damned if we stay
damned if we leave

David Brooks: The importance of staying with Iraq - Editorials & Commentary - International Herald Tribune