<<You would favor this course even if you could know that massacres worse then the killing fields of Cambodia would result? >>
Well, once again I have to say you have no proof whatsoever. NOBODY knows what will happen when you pull out. But you have no right to be there in the first place. Quite likely there will be a lot of killing and then one faction will rise to the top, one dictator will prevail over the other wannabes. It will not be the only dictatorship in the Middle East, most of the others are already paid for by your country anyway. This will be the only one you can look at with a clear conscience because you won't have to support it.
As for Ford and Cambodia, he was not threatened with a Pol Pot regime, but he was threatened by conservative forces with The Domino Effect, very solidly supported by all kinds of military and academic opinion, which, strangely enough, never came to pass. Just a lot of hot air in the end.
Did you ever hear the Biblical phrase, "Sufficient unto the day?" "Sufficient unto the day is the evil thereof." Meaning deal with the evil in front of you here and now - - don't be paralyzed by fear of what evils you will have to deal with tomorrow. TODAY there is an invasion and an occupation which are crimes at international law, which result in daily deaths of both Americans and Iraqis. Stop it. Get out. Observe the UN Charter which you are bound by law to observe. What you fear tomorrow may never come to pass. If it comes tomorrow, deal with it tomorrow. But for God's sake, don't let the fear of tomorrow's evil stop you from dealing with the shit that is going on right now.