The city of Babylon makes its second to the last appearance in historical sources after the fall of the Third Dynasty of Ur, which had ruled the city states of the alluvial plain between the rivers Euphrates and Tigris for more than a century. An agricultural crisis meant the end of this centralized state, and several more or less nomadic tribes settled in southern Mesopotamia. One of these was the nation of the Amorites ("westerners"), which took over Isin, Larsa, and Babylon.