It would appear that the Mitannian invasion of northern Mesopotamia and the Aryan invasion of India represented two streams of diverging migrations from a common cultural centre, and that the separate groups of wanderers mingled with other stocks with whom they came into contact. Tribes of Aryan speech were associated with the Kassite invaders of Babylon, who took possession of northern Babylonia soon after the disastrous Hittite raid. It is believed that they came from the east through the highlands of Elam. For a period, the dating of which is uncertain, the Mitannians were overlords of part of Assyria, including Nineveh and even Asshur, as well as the district called "Musri" by the Assyrians, and part of Cappadocia. They also occupied the cities of Harran and Kadesh. Probably they owed their great military successes to their cavalry. The horse became common in Babylon during the Kassite Dynasty, which followed the Hammurabi, and was there called "the ass of the east", a name which suggests whence the Kassites and Mitannians came. The westward movement of the Mitannians in the second millennium B.C. may have been in progress prior to the Kassite conquest of Babylon and the Hyksos invasion of Egypt. Their relations in Mesopotamia and Syria with the Hittites and the Amorites are obscure. Perhaps they were for a time the overlords of the Hittites. At any rate it is of interest to note that when Thothmes III struck at the last Hyksos stronghold during his long Syrian campaign of about twenty years' duration, his operations were directly against Kadesh on the Orontes, which was then held by his fierce enemies the Mitannians of Naharina.[295] During the Hyksos Age the horse was introduced into Egypt. Indeed the Hyksos conquest was probably due to the use of the horse, which was domesticated, as the Pumpelly expedition has ascertained, at a remote period in Turkestan, whence it may have been obtained by the horse-sacrificing Aryo-Indians and the horse-sacrificing ancestors of the Siberian Buriats. If the Mitanni rulers were not overlords of the Hittites about 1800 B.C., the two peoples may have been military allies of the Kassites. Some writers suggest, indeed, that the Kassites came from Mitanni. Another view is that the Mitannians were the Aryan allies of the Kassites who entered Babylon from the Elamite highlands, and that they afterwards conquered Mesopotamia and part of Cappadocia prior to the Hyksos conquest of Egypt. A third solution of the problem is that the Aryan rulers of the Mitannian Hittites were the overlords of northern Babylonia, which they included in their Mesopotamian empire for a century before the Kassites achieved political supremacy in the Tigro-Euphrates valley, and that they were also the leaders of the Hyksos invasion of Egypt, which they accomplished with the assistance of their Hittite and Amoritic allies.