=30.= INDIAN CONQUESTS. The first great expedition of Darius was against the Punjab´, or Five Rivers of Western India. The imperial revenues were increased one-third by the acquisition of this rich gold-tract, and a lucrative commerce now sprang up between the banks of the Indus and the shores of the Persian Gulf. =31.= SCYTHIAN CAMPAIGN. The next enterprise of Darius was against the Scythians of Central Europe, between the Don and the Danube. His design was to avenge the Scythian devastations of Media and Upper Asia a century before, and to terrify the barbarians into future good behavior by a display of his power; perhaps also to open a way into Greece by the conquest of the Thracian tribes. The whole army and navy of the empire, consisting of not less than 700,000 land soldiers and 600 ships, assembled at the Thracian Bosphorus, which they crossed by a bridge of boats constructed by Ionian engineers. The naval force was furnished wholly by the Greeks of the Ægean. =32.= Sending his fleet through the Euxine Sea into the Danube, with orders to make a bridge of boats two days’ journey from its mouth, Darius marched through Thrace, receiving or compelling the submission of its tribes, and adding their young men to his army. Arriving at the Danube, he crossed the bridge and gave orders to the Greeks to remain and guard it sixty days; if in that time he did not return, they might conclude that he had gone to Media by another route. The details of the great king’s operations north of the Danube are unknown to history. There were no great cities to take; the wandering Scythians destroyed their scanty harvests, stopped their wells, removed their families northward to places of security, and drew the invader after them into the depths of their forests or uninhabited deserts. Unable to bring his enemy to battle, and seeing his army reduced to great distress for want of food and water, Darius was compelled to retreat by the way he had come. The sixty days were more than elapsed when a Scythian force, which had been watching his movements, hastened to the Danube by a shorter route, urging the Ionians, who were still on guard, to destroy the bridge and leave Darius to perish, like Cyrus, in the northern deserts. The Greeks of Asia might thus have gained their freedom without a blow; but the tyrants who commanded the fleet had interests of their own quite separate from those of their people. Histiæ´us of Mile´tus urged upon his fellow-despots that their power must fall with that of Darius, being sustained by him against the popular will. His arguments prevailed, and the great king, arriving in the darkness of midnight, closely pursued by the Scythians, was able to repass the river in safety.