How did the Sumerians politically organize early Mesopotamia?

About 3000 B.C. the Sumerians moved into the area of
Mesopotamia in the area of the lower Tigris Euphrates River valley. There independent
city states, such as Ur, Erech, and Kish. Each city state had its own government and
ruler.  Each city state also had its own gods that they worshiped.  Sumerians believed
that they were dependent on their gods for their protection and food.  So their survival
depended on keeping the gods happy.  Only the priests knew how to do this and acted as
intermediaries between the gods and people. Therefore priests began to gain political
power until these city states were run by priest-kings.