I think this exchange between [host] David Gregory and Russ Feingold on this week's Meet the Press does a very good job of exposing the basic incoherence of the withdrawal-means-the-terrorists-have-already-won position:
MR. GREGORY: Not only has the president said that any kind of deadline for the withdrawal of U.S. troops is a mistake, but so have prominent members of your own party... Senator Hillary Clinton this February, the headline: "Hillary Rejects Deadline." "I don't think we should be setting a deadline. ...That just gives a green light to the insurgents and the terrorists, that if they just wait us out they can basically have the country. It's not in our interest, given the sacrifices we have made."
SEN. FEINGOLD: Well, of course, I haven't proposed a deadline. But, you know, the Democrats are making the same mistake they made in 2002, to let the administration intimidate them into not opposing this war, when so many of us knew it wasn't a good idea. And same thing with this taboo on talking about a timeline. It doesn't make sense. If the terrorists and the insurgents really thought that, why wouldn't they just stop blowing us up right now? Why wouldn't they just let us leave and then take over?
Russ is absolutely right. If the insurgency's aim was to eject us from Iraq so they could take over in our absence (though I've yet to see how they plan to do that), they wouldn't be hardening the Pharaoh's heart through bombings and IED's, they'd be laying quiet, letting things go smoothly so we'd pack up and go home and they could suit up and conquer their home.
I'd love to hear Hillary Clinton explain how the insurgents are going to go about having the country after our departure. The Iraqi constitution is being bogged down because Sunnis want access to the oil-rich areas within the Shi'ite territory. Do the math: the country is overwhelmingly Shi'ite, the oil is deep within the region they control, and the Sunnis need some of those profits to survive. So exactly how is this insurgency, which is being carried out by a fraction of a minority, going to overrun the country? And if their actions simply enrage the Shi'ites, as they will, doesn't it make sense that the rest of the Sunnis, who hold no demographic or economic cards, will be desperate for rapprochement and begin quelling the extremists in their own ranks?
So long as we're there, there's a buffer between the Iraqi people and those trying to destroy them. The insurgents may not be liked, but their actions can be rationalized as anti-imperialist warfare. Once we leave, they can't. Once we leave, the Sunnis need to figure out what strategies will put them in a sustainable position. Considering their percentage of the population and their geographic location, a rupture with the Shi'ites isn't going to survive that analysis.