It was against this region that Darius directed his first great aggressive effort.

Having explored the course of the Indus from Attock to the sea by means of boats, and obtained, we may suppose, in this way some knowledge of the country and its inhabitants, he led or sent an expedition into the tract, which in a short time succeeded in completely reducing it.

The Punjab, and probably the whole valley of the Indus, was annexed, and remained subject till the later times of the Empire.

The results of this conquest were the acquisition of a brave race, capable of making excellent soldiers, an enormous increase of the revenue, a sudden and vast influx of gold into Persia, which led probably to the introduction of the gold coinage, and the establishment of commercial relations with the natives, which issued in a regular trade carried on by coasting-vessels between the mouths of the Indus and the Persian Gulf.
The next important expedition--one probably of still greater magnitude--took exactly the opposite direction.

The sea which bounded the Persian dominion to the west and the north-west narrowed in two places to dimensions not much exceeding those of of the greater Asiatic rivers.