During this period of comparative peace, which may have extended from about B.C. 516 to B.C. 508 or 507, the general tranquillity was interrupted by at least one important expedition.  The administrational merits of Darius are so great that they have obscured his military glories, and have sent him down to posterity with the character of an unwarlike monarch—­if not a mere “peddler,” as his subjects said, yet, at any rate, a mere consolidator and arranger.  But the son of Hystaspes was no carpet prince.  He had not drawn the sword against his domestic foes to sheath it finally and forever when his triumph over them was completed.  On the contrary, he regarded it as incumbent on him to carry on the aggressive policy of Cyrus and Cambyses, his great predecessors, and like them to extend in one direction or another the boundaries of the Empire.  Perhaps he felt that aggression was the very law of the Empire’s being, since if the military spirit was once allowed to become extinct in the conquering nation, they would lose the sole guarantee of their supremacy.  At any rate, whatever his motive, we find him, after he had snatched a brief interval of repose, engaging in great wars both towards his eastern and his western frontier—­wars which in both instances had results of considerable importance.

The first grand expedition was towards the East.  Cyrus, as we have seen, had extended the Persian sway over the mountains of Affghanistan and the highlands from which flow the tributaries of the Upper Indus.  From these eminences the Persian garrisons looked down on a territory possessing every quality that could attract a powerful conqueror.  Fertile, well-watered, rich in gold, peopled by an ingenious yet warlike race, which would add strength no less than wealth to its subjugators, the Punjab lay at the foot of the Sufeid Koh and Suliman ranges, inviting the attack of those who could swoop down when they pleased upon the low country.  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.