after the end of the last Ice Age in 15,000-10,000 BCE and the first evidence of agriculture, the Sumerians arrived in Mesopotamia circa 5000 BCE. The early history of Mesopotamia under the Sumerians includes great cultural and technological advances. All of the Mesopotamian empires were largely collections of city-states with the one city with the best army determining domination in the region, until the decline of Mesopotamia in the first millennium BCE, when King Cyrus II (Cyrus the Great) conquered Mesopotamia by 539 BCE.
