To the north, or north-east, between Mesopotamia and the Caspian, there were gathered a congeries of restless tribes known to the Assyrians as the Madai or Matai, the Medes. They are mentioned first by Shalmaneser II. in 840, and few of his successors do not record campaigns against them. The earliest notice of them in the Old Testament is in connection with the captives of Samaria, some of whom in 720 were settled among them.[30] These Medes were probably of Turanian stock, but by the end of the eighth century, if we are to judge from the names of some of their chiefs,[31] their most easterly tribes had already fallen under Aryan influence, spreading westward from Persia.[32] So led, they became united and formidable to Assyria. Herodotus relates that their King Phraortes, or Fravartis, actually attempted the siege of Niniveh, probably on the death of Assurbanipal in 625, but was slain.[33] His son Kyaxares, Kastarit or Uvakshathra, was forced by a Scythian invasion of his own country to withdraw his troops from Assyria; but having either bought off or assimilated the Scythian invaders, he returned in 608, with forces sufficient to overthrow the northern Assyrian fortresses and to invest Niniveh herself. The other and southern group of peoples which threatened Assyria were Semitic. At their head were the Kasdim or Chaldeans.[34] This name appears for the first time in the Assyrian annals a little earlier than that of the Medes,[35] and from the middle of the ninth century onwards the people designated by it frequently engage the Assyrian arms. They were, to begin with, a few half-savage tribes to the south of Babylon, in the neighbourhood of the Persian Gulf; but they proved their vigour by the repeated lordship of all Babylonia and by inveterate rebellion against the monarchs of Niniveh. Before the end of the seventh century we find their names used by the prophets for the Babylonians as a whole. Assurbanipal, who was a patron of Babylonian culture, kept the country quiet during the last years of his reign, but his son Asshur-itil-ilani, upon his accession in 625, had to grant the viceroyalty to Nabopolassar the Chaldean with a considerable degree of independence. Asshur-itil-ilani was succeeded in a few years[36] by Sinsuriskin, the Sarakos of the Greeks, who preserved at least a nominal sovereignty over Babylon,[37] but Nabopolassar must already have cherished ambitions of succeeding the Assyrian in the empire of the world. He enjoyed sufficient freedom to organise his forces to that end.