Like many people sitting outside of Iraq, I have a hard time grasping the reasons for the violence. I understand that it is related to the ethnic cleansing that has gone on elsewhere, especially in Darfur, and that such activity is so common as to be considered a natural consequence of human nature.
As part of the research for my novel Lights Out, about the consequences of a large electromagnetic pulse over Hawaii, I've developed a simple model of behavior based on the "Big Five" personality traits defined by McCrae and Costa. In this model, everyone in the population has a "comfort zone" in each of the personality dimensions, corresponding to their personality. They attempt to influence their local environment to match their personality (for example, highly social people try to be around more people, and loners tend to stay away from people). To the extent that the environment does not support a person's comfort zone, the person will experience stress.
The model predicts that the situations with the highest stress on the most people involve high uncertainty about how to reach their comfort zones, coupled with an environment that is near one of the extremes. The uncertainty can be manifested in either ignorance of the environment, or ignorance of how the environment works (what to do to alter it). Conversely, the situations with the least stress involve high certainty and knowledge about the environment, which tends to be near the middle of the continuum of states (in the comfort zone of the average person in a random population).
Given the huge stress experienced by the people in Iraq, I can't help but wonder if my model's predictions apply to them. One huge source of uncertainty in their lives, we know, is that they don't know who to trust. Since other people are the most influential parts of our environment, it makes sense that this will be the greatest source of stress. To reduce that stress, they could be expected to latch on to people who have obvious, common traits (such as religion); and resist, if not destroy, those who don't. In terms of the personality dimensions, I would expect need for stability and accommodation to play the largest part, with extraversion close behind.
What does all this suggest about how to stop the bloodshed? For one, the sources of uncertainty have to be disabled; and my guess is that these sources are primarily low-accommodation people who have significant power over the others. In addition, a system for providing accurate information and understanding to the most people needs to be established; this should be the basic role of any central government, and might be manifested as a public education system. "Security" has been discussed a primary requirement, but I would argue that you can't enforce behavior that hasn't been properly defined or agreed to; this is where communication between people is critical, operating from a common set of facts.
One thing I do know is that people are least likely to attack others they identify with, and establishing a basis for community is the most important prerequisite for civilization.