Mesopotamia, "the land between the rivers," was the ancient Greek name for the triangular area between the Tigris and Euphrates rivers that stretched northward from a point a little above modern Baghdad to the mountains of Armenia. In modern usage, however, in reference to antiquity, the term Mesopotamia refers to most of what is now Iraq. This broader definition--the one used here--adds to the original territory the land east of the Tigris (ancient ASSYRIA) and the lower Tigris-Euphrates Valley from Baghdad to the Persian Gulf (ancient BABYLONIA).

Some parts of Mesopotamia were sparsely inhabited--and others not at all--until after about 8000 BC, when the domestication of plants and animals brought about an agricultural revolution. This key step in the development of human civilization made possible an increased food supply and an accompanying growth of population and allowed nomads and cave dwellers to become farmers and herders. People began to move down from the mountains to the grassy uplands and well-watered plains of northern Mesopotamia. By 6000 BC primitive villages stretched from Assyria along a fertile strip just below the Armenian hills to the Euphrates River and beyond.

Later, about 5000 BC, the occupation of the lower valley (Babylonia) was begun by people who started at the Persian Gulf and gradually moved upriver. Because of inadequate rainfall the inhabitants of the lower valley had to resort to irrigation in order to farm the land. Irrigation required a highly organized governmental structure to mobilize and direct the efforts of the workers. At the same time, a lack of stone, wood, metals, and other commodities led the people of the southern valley to develop industry in order to produce goods that could be traded for the materials needed and to develop an extensive trade with the outside world. The southern valley's more complex economy, coupled with its more productive agriculture, induced a more sizable increase in population and brought the rise of large villages, even cities. Thus by 3100 BC civilization may be said to have begun in southern Mesopotamia, the area called SUMER.

During the Sumerian period, which lasted until about 2000 BC, the Sumerians organized a system of flood control and a pattern of irrigation and created an enduring writing system (cuneiform), religious literature, architectural form, and economic organization.

In the centuries after 3100 BC, Sumerian civilization was borrowed and adapted by the people of northern Mesopotamia as well as by those in the region now called Iran and by countries west of the Euphrates bordering on the Mediterranean Sea. Mesopotamia, unlike Egypt, was a land open to invasion, and therefore as time passed the country came under the political domination of a succession of conquerors. These people often enriched Mesopotamian culture as they introduced their traditional beliefs, practices, and customs. However, because of environmental conditions peculiar to Mesopotamia and the existence of an already complex civilization there, these later peoples adopted Sumerian culture as the basis for their cultural innovations.

During the Sumerian period Semitic-speaking tribes from the west occupied the northern part of the lower valley (AKKAD) around Babylon and set up the so-called Sargonid dynasty (c.2350-2100 BC) that briefly controlled the Sumerians in the south. More important, however, were the AMORITES, a larger Semitic group that came shortly after 2000 BC. These people, with their capital at BABYLON, established the Old Babylonian Kingdom that unified the whole valley. The Amorites were numerous enough to make their language the prevailing speech in Babylonia and to merge their culture with that of the Sumerians to create a new literature, a different political organization, advances in sculpture, and a reorganization of commercial procedures. In addition, they gave an impressive maturity to the elementary mathematics and astronomy developed by the Sumerians.

About the same time that the Amorites came to Babylonia, another, related Semitic group, the Assyrians, took over the eastern part (Assyria) of northern Mesopotamia. Although they, too, borrowed from the culture of the Sumerians, they proceeded more slowly than the Amorites, and Assyrian civilization flowered at a later time. Both the Babylonians and Assyrians were in political eclipse between 1550 and 1100 BC as invaders from the north dominated their respective territories: Mitannians (see MITANNI) in Assyria and KASSITES in Babylonia. By about 1100, the Mitannian and the Kassite regimes had crumbled and disappeared without having materially affected Mesopotamian culture, and the Babylonians and Assyrians rose to new political and cultural heights.

Between 745 and 612 BC the Assyrians built up and then lost a great empire that encompassed Mesopotamia, Syria, Palestine, and Egypt, with tributary nations in western Iran as well. Assyrian civilization, an elaboration of the basic Mesopotamian, was the wonder of the age. After the fall of Assyria, Babylonia under the Neo-Babylonian (or Chaldean) kings--Nabopolassar (r. c.630-605), NEBUCHADNEZZAR II (r. 605-561), and others--flourished until the Persian conquest of 539 BC. Henceforth Mesopotamia was incorporated in foreign empires: the Persian, Seleucid, Parthian, Sassanian, Arab, and Ottoman. In 1921, following World War I, Mesopotamia was constituted as the new state of Iraq under British mandate. It became independent in 1932.

Ancient Mesopotamia bequeathed to the Western world a great legacy. Today we still use Assyrian words for many plants and minerals; the Babylonians were the founders of our algebra and astronomy; the sexagesimal system of the Sumerians appears in our astronomy and timekeeping (60 seconds, one minute; 60 minuts, one hour); and the Mesopotamian influence on the authors of the Old Testament is well known.