It has long been argued that withdrawal of U.S. troops would destabilize Iraq and the region. It would be seen as a defeat for the U.S., and would trigger invasion or proxy-invasion by neighboring countries (Iran for the Shia, Syria and Saudi Arabia for the Sunni). Even those supporting withdrawal do so primarily from a feeling that our presence is only puting off the inevitable. There is no solution, and risking American lives and squandering American resources to no purpose makes no sense.
I will now make the affirmative case for withdrawal as follows: continued U.S. occupation is a moral hazard for the Iraqi political elite. If the U.S. announced withdrawal by a date certain, and took active steps to that effect, it would encourage national reconciliation.
Argument below cut.
For those unfamiliar with the term, a "moral hazard" refers to a situation where a party is encouraged to take actions that are contrary to desired actions, usually (but not always) because the party is insulated from the consequences of its actions.
Here, the presence of the U.S. creates a dynamic that warps behavior. The problem is that each party recognizes that it must make concessions, but wants to minimize the concessions it makes. This can be achieved, each party hopes, by manipulating the United States, to force the other party to make concessions. Further, each party is aware that they are insulated by the United States from the consequences of their actions (to the extent overall security is maintained). There is no possibility of a new election until the current government adopts a constitution and resolves outstanding issues -- so the elected leaders cannot be deposed by their own people via democratic processes. There is no possibility of military destabilization, because the U.S. will prevent the current government from falling. The U.S. will also provide the highest level of security possible to elected leaders.
Nor will the U.S. itself dissolve the government. It can't. To do so would be to undermine the essential credibility of the government, because the U.S. has billed this as the democratically elected government of the Iraqi people.
The result is to create a situation where none of the elected political leaders has incentive to make the necessary concessions to achieve political unity -- because the leaders are all insulated from the refusal to concede. It is further complicated because as long as none of the Iraqi leaders compromises, no single one of them will be held accountable by his or her constituency. OTOH, if any leader does move first to make a concession, that leader's constituency is likely to consider him or her weak -- especially if there is no concurrent concession from other parties.
If the U.S. forces announce an independent date for withdrawal, however, it radically changes the incentives of the leaders (assuming they believe the schedule). Suddenly, all parties will seek to gain maximum advantage during the interim period before withdrawal, because the outcome of withdrawal is entirely unforeseeable. How to create "maximum advantage" is unclear. It may result in parties hardening their positions and focusing on manipulating the U.S. before withdrawal. It may break down into an effort to create military advantage in anticipation of military struggle for dominance after U.S. departure. These are the results feared by the "withdrawal=increased instability" people, and such concerns are not without merit.
But it is also plausible that the fear of this instability will push leaders to make concessions and contract broader alliances -- which they have failed to do. Many of these leaders must worry that in any disintegration of social order, they (both personally and their constituency) will do poorly. No faction has sufficient strength to be sure of forcing its will on another, as even Shia are not a united block. As a consequence, the certainty of U.S. withdrawal will potentially drive the existing leadership to preserve stability and have the stable government blessed by the U.S.
This seems an awful risk. But the consequence is an Iraqi government with no incentive to make difficult political concessions and every rational reason not to do so. If we want to force peace, we need to risk civil war. Otherwise, we end up staying forever in the limbo of the damned, hated by everyone while struggle drags on just enough to preserve the status quo.