I'm not claiming the following model is true, but it is worth seeing what the simplest public choice model implies...
Dictatorships are generally most brutal when the fear of being overthrown is strongest. The most benevolent dictatorships, in relative terms, tend to have strong roots in the country’s social and economic power centers. This would help explain, for instance, why the minority Sunni Saddam Hussein was so tyrannical against his potential opponents. Without extreme oppression, he would have lost power and his life.
The optimistic scenario for Iraq was (way back when) that a Shiite autocracy, with broad-based public support, would be considerably less brutal. Once in power, the ruling clique would find it much easier to stay in power without extreme brutality. At least that is how the theory went.
In this view, the critical U.S. mistake was not disbanding the (largely Sunni) army, which was in any case inconsistent with the best available power structure. The critical mistake was creating a government that had no real unity and no real chance of having power on the ground.
The pessimistic scenario is that there are no broad-based constituencies left, or perhaps there never were any in the first place. Under the former case American policy has been far more harmful, in net terms, than under the latter case. It is possible that our handling of the transition disbanded whatever broad-based groups were in place to eventually rule. Or perhaps Saddam had already destroyed them.
Partition has a certain logic in this model. But there is no one to effectively oversee the process of division and allocation, either for the population or the oil. I would expect a good million or half million lives to be lost from the resulting slaughter and the forced migrations of population.
To repeat, I am not claiming this model is true. But if it is false, it is worth thinking about what further assumptions should be added or which current assumptions should be dropped.
Addendum: Modeling the current Iraq is difficult for a few reasons. It is rare for an occupying power to set up a democracy, so historical data are scarce. In any case this is not the world of MacArthur and postwar Japan. Nor is it the democracy of Anthony Downs or Arendt Lijphart. For many unusual governmental forms, I start with the implicit models of Gordon Tullock’s Autocracy and the problems of stability and cycling autocratic coalitions. But Iraq seems too far from stability for cycling to be the major problem. The instability seems radically overdetermined, and that makes comparative statics difficult.
The closest parallel I can think of is Yugoslavia in the early 1990s, when relative stability gave way to bloodshed. Fear encouraged a mental overinvestment in strategies of ethnic solidarity and many groups started launching pre-emptive attacks, leading to widening circles of violence and then greater fear.
There are many smart writers on Iraq, with varying degrees of knowledge and information. I wish more of them would seek to provide a simple model of what is going on.
If you do leave comments, please focus on public choice issues rather than attacking or defending the war itself.