Iraq is disintegrating because there never has been a NATION of Iraq. A nation is not a piece of dirt, it is a PEOPLE. Like the former Yugoslavia, "Iraq" is an illusion, a chimeric monster stitched together by British diplomats. It could only be held together by a blood-thirsty dictator or by military occupation.
There are three nations existing within the borders of what we call Iraq. As long as they are forced to live under one government, they will fight politically to control that government or rebel militarily if they fail.
Like the Serbs, Croations, and Bosnians of "Yugoslavia", the Kurds, Shiites, and Sunni of "Iraq" will only be able to peacefully co-exist when they are allowed to have countries of their own.