The Mesopotamians lived in the region between the Tigris and Euphrates Rivers in modern-day Iraq. They relied on these rivers for irrigation and grew crops like wheat and barley. In the third millennium BC, King Hammurabi united several Mesopotamian nations into a single kingdom based in Babylon. Babylon was a major city with royal palaces, temples, and defensive walls. The Mesopotamians believed many gods controlled different aspects of life and made offerings to them. They developed one of the earliest systems of writing using symbols on clay tablets.