The first argument is that we are a big part of problem. Iraqis see us as occupiers and want us gone, if not dead. If we leave the Iraqi government will be forced to make the compromises necessary to avert all-out civil war and national reconciliation will follow.
This is an unpersuasive case. The National Intelligence Estimate indicates a likelier outcome:
If coalition forces were withdrawn rapidly during the term of this estimate, we judge that this almost certainly would lead to a significant increase in the scale and scope of sectarian conflict in Iraq, intensify Sunni resistance to the Iraqi government, and have adverse consequences for national reconciliation.However, there is a better argument for withdrawal. It says that we have a poor record of affecting events in this part of the world. We overthrew an Iranian democracy in the fifties and paid for it twenty years later when Khomeini took power there. We supported the Muhajadeen in Afghanistan only to see them morph into the terrorist-sheltering Taliban. We aided Saddam Hussein as a buttress against Iran only to see him emerge as an expansionist threat. We invaded Iraq to install a democratic regime and we got... what we got. Unintended adverse consequences have been more common than successful applications of our will in this area of the world. Moreover, the cost of continuing our Iraqi presence is huge and the situation is complex. Whatever the adverse consequences of our absence might be, we have no assurance that our presence will have a positive effect. We should just butt out while we’re behind.
I’m not sure I’m persuaded by this view either, but it is a respectable argument that cannot be dismissed out of hand.