It appears to have been a portion of the Empire in the reign of Darius Hystaspis, but whether that monarch, or Cambyses, or the great founder of the Persian power conquered it, cannot at present be determined.
The conquest of the vast tract lying between the Caspian and the Indus, inhabited (as it was) by a numerous, valiant, and freedom-loving population, may well have occupied Cyrus for thirteen or fourteen years.
Alexander the Great spent in the reduction of this region, after the inhabitants had in a great measure lost their warlike qualities, as much as five years, or half the time occupied by his whole series of conquests.

Cyrus could not have ventured on prosecuting his enterprises, as did the Macedonian prince, continuously and without interruption, marching straight from one country to another without once revisiting his capital.

He must from time to time have returned to Ecbatana or Pasargadae; and it is on the whole most probable that, like the Assyrian monarchs, he marched out from home on a fresh expedition almost every year.

Thus it need cause us no surprise that fourteen years were consumed in the subjugation of the tribes and nations beyond the Iranic desert to the north and the north-east, and that it was not till B.C.
539, when he was nearly sixty years of age, that the Persian monarch felt himself free to turn his attention to the great kingdom of the south.
The expedition of Cyrus against Babylon has been described already.
Its success added to the Empire the rich and valuable provinces of Babylonia, Susiana, Syria, and Palestine, thus augmenting its size by about 240,000 or 250,000 square miles.

Far more important, however, than this geographical increase was the removal of the last formidable rival--the complete destruction of a power which represented to the Asiatics the old Semitic civilization, which with reason claimed to be the heir and the successor of Assyria, and had a history stretching back for a space of nearly two thousand years.