The basic problem with pursuing any version of partition today in Iraq is that it is probably impossible to do so without either causing the all-out civil war in the first place, or deploying the hundreds of thousands of American and other first-world troops whose absence has been the first-order problem preventing reconstruction from succeeding. Other than the Kurds, few Iraqis--whether political leaders, militia commanders or ordinary citizens--want their country divided. And many of those who are fleeing their homes are not merely peacefully resettling in a more ethnically homogeneous region, but are joining vicious sectarian militias like the Mahdi Army in hope of regaining their homes or at least extracting revenge on whoever drove them out.On the other hand, Byman predicts that the "biggest headache of all" will be how to prevent Iran from "intervening" in Iraq. But why would the Iranian government even want to stick its hand into that particular bear trap? And if they're stupid enough to try, why should we bother to stop them? (Serious question.)
Nor is it clear that a move to partition would result in the neat division of Iraq into three smaller states, as many of its advocates seem to assume. As noted above, the Sunnis and the Shi'a are highly divided and are likely to fight amongst themselves, leading to regular war within the communities and a probable fracturing of power in areas where they predominate. Many militia leaders, particularly the Sadrists, have made clear that they intend to fight for all of the land they believe is "theirs", which seems to include considerable land that the Sunnis consider "theirs." Baghdad is one area of contention between Sunnis and Shi'a, but many major cities are also home to multiple communities. Much of Iraq's oil also lies in areas that are not peopled exclusively by one group.
The partition model most observers seem to have in mind is the former Yugoslavia. There, however, years of fighting preceded the partition, clarifying the relative balance of power of the parties involved. Perhaps more important, the communities had a degree of unity and clear leaders--Slobodan Milosovic and Franjo Tudjman, for example--who could command their followers to stop the fighting. Nuri al-Maliki and other Iraqi leaders cannot issue similar orders even if they wanted to. Iraq's civil war is just not yet "ripe" for a solution like partition, and therefore to impose it upon Iraq would require a far greater military commitment by the United States than the present one--closer to the troop to population ratio required to police the Bosnia partition, where the conflict actually was ripe for solution when Richard Holbrooke sat down at the negotiating table in Dayton.