What Would Happen if the U.S. Military Left Iraq?

There is a lot of debate as to whether American forces should be returned from their mission in Iraq. Our Congress has been debating this for months now and most of it seems to be centered around presidential politics for the upcoming election. There doesn't seem to be a whole lot of time spent on addressing what would happen if we pulled.

One commentator has ventured to start a bit of a discussion on the potential scenarios we can expect from a troop withdrawal. I personally believe that this side of the discussion needs to be had, regardless of how one feels about the veracity of the war.

The question for me is not whether we should withdraw, as eventually we will, but under what circumstances? What is our responsibility to the American public, the Iraqi people and the overall issue of addressing terrorism, despotism, fascism and general human suffering in the world?