I, along with most of the world, have always thought the invasion of Iraq was a damn fool idea, but now you've done it, the consequences of withdrawl have to be thought through.
If the coalition forces withdraw in the face of what looks increasingly like civil war, the U.S. will lose all credibility. The U.K. already has, thanks largely to an open microphone. The little man in Tehran will crow so loudly that we'll be able to hear him from here.
His first step will be to take over Southern Iraq.
What happens after that, I can't guess. There are several scenarios; all dire. The very best is a nuclear-armed Iran. The others are worse.
What to do? Sorry, but I have no idea, except to tough it out with massive numbers of troops on the ground. It might be advisable to cut Iraq into 3, but then the bits could be picked off by Iran and Syria. The whole situation is a huge mess.
To cap it all off, Western nations are financing it all by buying Iranian oil.
We have no money; we must use our brains. -Ernest Rutherford.