Sargon was the world's first empire-builder, sending his troops as far as Egypt and Ethiopia. He attempted to establish a unified empire and to end the hostilities among the city-states. Sargon's rule introduced a new level of political organization that was characterized by an even more clear-cut separation between religious authority and secular authority. To ensure his supremacy, Sargon created the first conscripted army, a development related to the need to mobilize large numbers of laborers for irrigation and flood-control works. Akkadian strength was boosted by the invention of the composite bow, a new weapon made of strips of wood and horn.

Despite their military prowess, Akkadian hegemony over southern Mesopotamia lasted only 200 years. Sargon's great- grandson was then overthrown by the Guti, a mountain people from the east. The fall of the Akkadians and the subsequent reemergence of Sumer under the king of Ur, who defeated the Guti, ushered in the third phase of Sumerian history. In this final phase, which was characterized by a synthesis of Sumerian and Akkadian cultures, the king of Ur established hegemony over much of Mesopotamia. Sumerian supremacy, however, was on the wane. By 2000 B.C. the combined attacks of the Amorites, a Semitic people from the west, and the Elamites, a Caucasian people from the east, had destroyed the Third Dynasty of Ur. The invaders nevertheless carried on the Sumero-Akkadian cultural legacy.