Without there being some repercussions, there will be more infighting, some people will die, others will go to jail, however the US stuck it's foot into this and there is no pretty way to get out (should have finished off afghanistan first, captured Osama and then head out to Iraq, but that's a different story). But they have to pull out the troops properly, in a way that will allow the iraqies to defend themselves as well, not just leave them completely open to hostile attacks.