But Assyria was not the only power which Babylonia had to guard itself against. On its eastern frontier Elam was also panting for expansion. Its chief caravan roads ran from Susa through Assyria towards Asia Minor, and through Babylonia towards the Phoenician coast. It was probably because its commerce was hampered by the growth of Assyrian power in the north, as Servia's commerce in our own day has been hampered by Austria, that it cherished dreams of conquering Babylonia. In fact, as Kassite influence suffered decline, one of the great problems of international politics was whether Elam or Assyria would enter into possession of the ancient lands of Sumer and Akkad. Ashur-uballit's vigorous policy of Assyrian expansion was continued, as has been shown, by his son Bel-nirari. His grandson, Arik-den-ilu, conducted several successful campaigns, and penetrated westward as far as Haran, thus crossing the Babylonian caravan road. He captured great herds of cattle and flocks of sheep, which were transported to Asshur, and on one occasion carried away 250,000 prisoners. Meanwhile Babylonia waged war with Elam. It is related that Khur-batila, King of Elam, sent a challenge to Kurigalzu III, a descendant of Kadashman-Kharbe, saying: "Come hither; I will fight with thee". The Babylonian monarch accepted the challenge, invaded the territory of his rival, and won a great victory. Deserted by his troops, the Elamite king was taken prisoner, and did not secure release until he had ceded a portion of his territory and consented to pay annual tribute to Babylonia. Flushed with his success, the Kassite king invaded Assyria when Adad-nirari I died and his son Arik-den-ilu came to the throne. He found, however, that the Assyrians were more powerful than the Elamites, and suffered defeat. His son, Na´zi-mar-ut´tash[409], also made an unsuccessful attempt to curb the growing power of the northern Power. These recurring conflicts were intimately associated with the Mesopotamian question. Assyria was gradually expanding westward and shattering the dreams of the Babylonian statesmen and traders who hoped to recover control of the caravan routes and restore the prestige of their nation in the west. Like his father, Adad-nirari I of Assyria had attacked the Aramaean "Suti" who were settling about Haran. He also acquired a further portion of the ancient kingdom of Mitanni, with the result that he exercised sway over part of northern Mesopotamia. After defeating Na´zi-mar-ut´tash, he fixed the boundaries of the Assyrian and Babylonian spheres of influence much to the advantage of his own country.