Zeyad of Healing Iraq has been working on a multi-part history of the Iraqi tribes (part one, part two, part three) which is definitely worth your time and consideration if you have any interest in the whole tangled tribal mess in Iraq. My understanding of Iraqi history is shallow at best, and I learned a lot. The one thing that sticks out for me is just how young Iraq is as a nation. There tends to be a lot of bosh talked about Ancient Mesopotamia, but one thing that Zayed's story of repeated tribal migrations from the south and west is how the rural and urban areas of Iraq were constantly dying, and being replaced by desert nomads. Cities in the pre-modern period were black holes in even the healthiest of environments - they suck in the peasantry from their surroundings, while the city-dwellers die of innumerable ills, and the surviving children of the peasantry, now city-dwellers themselves, are killed off in turn by later plagues, famines, warfare, and the like. For all of Saddam's pretensions of being the new Sargon, the new Nebeccanezzer, son of Nineveh and Babylon, modern Iraq is a thoroughly Arabized society descended primarily from nomad populations.
The cities become tribalized, as the tribes became Shia-fied, and then replaced by new Sunni and then Wahhabi migrants from the desert fringes. A further interesting point is how the southern cities outside of Basra, Baghdad, and the shrine cities are Ottoman foundations of the 19th century, and not ancient at all. Fascinating reading.