"From earliest recorded times, Nippur was a sacred city, not a political capital. It was this holy character which allowed Nippur to survive numerous wars and the fall of dynasties that brought destruction to other cities. Although not a capital, the city had an important role to play in politics. Kings, on ascending the throne in cities such as Kish, Ur, and Isin, sought recognition at Ekur, the temple of Enlil, the chief god of the Mesopotamian pantheon (Fig. 3). In exchange for such legitimization the kings lavished gifts of land, precious metals and stones, and other commodities on the temples and on the city as a whole. At the end of successful wars, rulers would present booty, including captives, to Enlil and the other gods at Nippur. Most important, kings carried out for the city elaborate construction and restoration of temples, public administrative buildings, fortification walls, and canals. Even after 1800 B.C., when the Babylonians made Marduk the most important god in southern Mesopotamia, Enlil was still revered, kings continued to seek legitimization at Nippur, and the city remained the recipient of pious donations. The city underwent periodic declines in importance [Gibson 1992) but rose again because its function as a holy center was still needed. The greatest growth of the city (Fig. 2), which occurred under the Ur III kings (c. 2100 B.C), was almost matched in the time of the Kassites (c. 1250 B.C.) and in the period when the Assyrians, from northern Iraq, dominated Babylonia (c. 750-612 B.C.)."

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http://asmar.uchicago.edu/OI/PROJ/NIP/PUB93/NSC/NSC.html