In about 3500 B.C., a people known as the Sumerians migrated from Asia into Mesopotamia. These people founded a city located in the Sumer Valley.
By around 3000 B.C., the Sumerians had formed a number of complex city-states. A city-state is a large city along with any surrounding territory that is ruled by a single entity or ruler. It is believed that there were at least 12 Sumerians City-States in the Fertile Crescent.
The people of these city-states shared a common culture and religion with one another. However, their rulers had complete autonomy from one another. They ruled their city-states without any interference from the leaders of other city-states.