What Kind Of Government Did Mesopotamia Have?

We know very little about Mesopotamia because it has been a war zone for a long time. The name means "between the rivers", and the rivers are Tigris and Euphrates. We call the people Sumerian, but we have no idea how many tribes or what period of time that might include. There are a little over a thousand cities buried in dust and only a few have been excavated. Population might have been about two million (a couple thousand per city), and they had no preparations for war, no self defense at all. And that is all we know for sure at this time.