This movement did not suffer an interruption through the invasion of the Cassites. Though Nippur, rather than Babylon, appears to have been the favorite city of the dynasty, the course of civilization flows on uninterruptedly, and it is not until the growing complications between Babylonia and Assyria, due to the steady encroachment on the part of the latter, that decided changes begin to take place. About 1500 B.C. the first traces of relationship between Babylonia and the northern Mesopotamian power, Assyria, appear. These relations were at first of a friendly character, but it is not long before the growing strength of Assyria becomes a serious menace to Babylonia. In the middle of the thirteenth century, Assyrian arms advance upon the city of Babylon. For some decades, Babylon remains in subjection to Assyria, and although she regains her independence once more, and even a fair measure of her former glory, the power of the Cassites is broken. Internal dissensions add to the difficulties of the situation and lead to the overthrow of the Cassites (1151 B.C.). Native Babylonians once more occupy the throne, who, although able to check the danger still threatening from Elam, cannot resist the strong arms of Assyria. At the close of the twelfth century Tiglathpileser I. secures a firm hold upon Babylonia, which now sinks to the position of a dependency upon the Assyrian kings. V. In contrast to Babylonia, which is from the start stamped as a civilizing power, Assyria, from its rise till its fall, is essentially a military empire, seeking the fulfillment of its mission in the enlargement of power and in incessant warfare. Its history may be traced back to about 1800 B.C., when its rulers, with their seat in the ancient city of Ashur, first begin to make their presence felt. The extension of their power proceeds, as in Babylonia, from the growing importance of the central city, and soon embraces all of Assyria proper. They pass on into the mountain regions to the east, and advancing to the west, they encounter the vigorous forces of Egypt, whose Asiatic campaigns begin about the same time as the rise of Assyria. The Egyptians, abetted by the Hittites--the possessors of the strongholds on the Orontes-- successfully check the growth of Assyria on this side, at least for a period of several centuries. In the meanwhile, the Assyrian king gathers strength enough to make an attack upon Babylonia.