First, I'm wondering what you think the result of an American withdrawal would be? And we really have to ask that about two spheres, the internal Iraqi results, and the effect on America's security.
There will be a bloody fight within Iraq for control of that nation. Whether that will be bloodier than the current civil war, I can't say. But it will be a bloody confrontation without the added carnage of American troops. On the domestic front, it will be better to have our troops not playing - essentially - pattycake with Iraqi forces who would just as soon betray them. Instead we would be back on track hunting down Al Qaeda and their affiliates instead of nation building. The postwar plans, if you can call them that, thought we would prop up a guy like Chalabi and Iraq would have some kind of democracy (everybody remember Bush's second inaugural speech?). That isn't going to happen. They want Sharia law. They don't want a secular, progressive republic. They'll pick what they want.
One of my questions is, who are the "they" that want Sharia law - a majority of Iraqis, or a majority of the Iraqis with guns?
If the civil war is truly intractable the answer may not matter much, but it seems to be worth knowing whether we failed because we were outfought or outvoted.
As to the notion that we will witness a violent civil war without intervention from Turkey, Iran, or Saudi Arabia - who knows?
Bush has created a disaster and I have no confidence in his judgment as to a solution, but I am not exactly sold on cut-and-run, either.