I think that the US has a very clear motive for trying to convince everyone that it there would be civil war if they pulled out. Now, I'm not dismissing the possibility, but its rather clear that the civil war scenario stinks of politics opportunism.
What do I think would happen if the US (please don't refer to it as a coalition aldo, since you know damn well that its not) pulled out?
I honestly couldn't say. Civil war is one possibility, I'm not denying that, but I don't pretend to know enough about the complex internal politics of Iraq, especially after the past 16 months, which have been particularly unusual, to say for sure.
But what I can say with some certainty is that, at least as far as I can see, the US troops are *causing* the instabilty, not guarding against it. Right now, 100% (or very close to it) of the fighting is between the US plus Iraqi police (collaborators let call them) and various insurgent groups. Now, if the US left, would these groups break their alliance and turn on each other? Maybe, maybe not. The US is a unifying force, in that it unites groups with, I would guess, rather different views and policies, against the occupation. It could very well be that when you take this unifying force away, it would be an all out battle for who will rule, but I don't think thats certain.
But this is all working under the assumption that the US will stay only so long as it takes "to do the job", which is until elections, which clearly is not the case.