The war was stupid and wrong from the start. We (Americans, British, etc.) went into it on false pretences, and we have almost certainly done more harm than good. But we are there now. You can't just charge into a neighbor's house and make a mess and then run off. We have made ourselves reponsible for what happens. To withdraw unilaterally simply compounds the evil we committed by invading in the first place.
We must withdraw when and only when it is clearly the desire of the Iraqis as a whole that we do so. I would applaud Congress if it explored the question of whether Bush should be impeached for his dubiously constitutional actions in the conduct of the war, but it ought not and must not impose a timetable for withdrawal. Such an action will only embolden the insurgents. We had no right to invade in 2003, and we have no right to leave now. We can leave only when we are unambiguously told to do so by the Iraqis. It's no longer our decision.