This is from Zeynep at Under the Same Sun from last month, but it hasn't gotten any less accurate since then:
It's obvious to everyone that U.S. forces are not contributing to the security situation in the country, to say the least. While I'm well aware that there are internal forces that might continue killing Iraqis even after a withdrawal, I don't see how that would justify the continued presence of a force that is constantly bombing and shooting and is utterly unwilling, unable or uninterested in carrying out any kind of policing activity -- which is what Iraq needs right now.
The longer we stay, the worse shape that country is going to be when we do leave. And the longer we stay, the more dead there will be, on all sides.
Between the events of last spring and summer, the door on the possibility that the people of Iraq would consent to a U.S. occupation has closed. Only two related questions remain: what will we make them pay in order to stay and what will they try to make us pay to get us to leave. Every passing day adds to the tally of both answers.