If we pulled out and left Iraq to a civil war, I very much doubt that an al-Queda faction would end up on top. Even if the Sunnis generally joined forces with the foreign fighters and indigenous Sunni radicals, it's not clear they'd win given the Shia majority, the fact that weapons are now pretty dispersed into the Shia population, and that Iran would certainly help the Shia vs a Sunni radical faction. Whether an al-Queda / former regime elements alliance would last that long after a U.S. departure is also quite open to question.

Unless of course you are arguing that there is simply an "us and good Iraqis" vs "them, you know all of them, those 'terrorists'". Well, that kind of simplistic thinking is no more likely to win Iraq than Kerry's resolution, and fortunately, most of our military planners are seeing the complexity of the situation.

Note that I'm not saying the civil war I describe is a desirable option, and though there wouldn't have been anything like the U.S. handing over the keys to the city to Zarqawi, an Iraq that messed up would certainly have opened up spaces for forces whose primary focus is anti-Western (vs primarily focussed on taking over Iraq) to operate, and that would be bad.