The war in Iraq has been continually evolving. It has not been one mission, but rather a series of missions.
Mission 1: Overthrowing Saddam Hussein and the Baathist gangsters. This mission has been accomplished.
Mission 2: Creating a constitution and electing a democratic government. This mission has been accomplished.
Mission 3: Preventing Al Qaeda from overthrowing the democratically elected government. This mission has been mostly accomplished. What’s left of Al Qaeda still has a stronghold in Anbar province, but is no longer a serious threat to the government, and defeating them in Anbar is a mission that can be accomplished, as it was in Fallujah and elsewhere.
Mission 4: Preventing the sectarian violence between the Shia and the Sunnis from plunging Iraq into civil war. This mission has not been accomplished, and it is an open question whether it can be, and even whether it really is our mission.
None of these missions have been easy, nor have any of their outcomes been at all certain. Most critics of the war have consistently maintained, before the fact, that they were all impossible, foolhardy undertakings doomed to failure. As each mission has been accomplished, the critics have forgotten it and moved on to the next. At each stage of the war, it has been painted as a lost cause from which we should extricate ourselves as quickly as possible.
Mission 4 is the toughest nut of them all. The more killing there is, the more it tends to escalate. The Mahdi Army of Muqtada al-Sadr is being funded, equipped, and trained by Iran, and has become an increasingly formidable force, which has zero interest in a peaceful, unified, democratic Iraq. It is also, unfortunately, the main power base of the current Maliki government. If the U.S. withdraws from Iraq, whether quickly or slowly, the most likely result will be the extermination of the Sunnis and the establishment of Iraq as a client state of Iran. Preventing such a result would seem to be in our interest and worth a considerable effort. Success is not certain, as it has not been every step of the way, but then, in war it rarely is.