At one blow the region of northwestern India known as the Punjab, was brought under Persian authority; and thus with a single effort were the eastern limits of the empire pushed out so as to include one of the richest countries of Asia--one which henceforth returned to the Great King an annual revenue vastly larger than that of any other province hitherto acquired, not even excepting the rich district of Babylonia. With an army numbering, it is said, more than 700,000 men, Darius now crossed the Bosphorus by means of a sort of pontoon bridge, constructed by Grecian architects, and passing the Danube by means of a similar bridge, penetrated far into what is now Russia, which was then occupied by Scythian hordes. The results of the expedition were the addition of Thrace to the Persian empire, and the making of Macedonia a tributary kingdom. Thus the Persian kings secured their first foothold upon the European continent. The most significant campaign in Europe was yet to follow. In 500 B.C., the Ionian cities in Asia Minor subject to the Persian authority revolted. The Greeks of Europe lent aid to their sister states. Sardis was sacked and burned by the insurgents. With the revolt crushed and punished with great severity, Darius determined to chastise the European Greeks, and particularly the Athenians, for their insolence in giving aid to his rebellious subjects. Herodotus tells us that he appointed a person whose sole duty it was daily to stir up the purpose of the king with the words, "Master, remember the Athenians." A large land and naval armament was fitted out and placed under the command of Mardonius, a son-in-law of Darius. The land forces suffered severe losses at the hands of the barbarians of Thrace, and the fleet was wrecked by a violent storm off Mount Athos, three hundred ships being lost (492 B.C.). Two years after this disaster, another expedition, consisting of 120,000 men, was borne by ships across the AEgean to the plains of Marathon. The details of the significant encounter that there took place between the Persians and the Athenians will be given when we come to narrate the history of Greece. We need now simply note the result,--the complete overthrow of the Persian forces by the Greeks under Miltiades (490 B.C.). Darius, angered beyond measure by the failure of the expedition, stirred up all the provinces of his vast empire, and called for new levies from far and near, resolved upon leading in person such an army into Greece that the insolent Athenians should be crushed at a single blow, and the tarnished glory of the Persian arms restored. In the midst of these preparations, with the Egyptians in revolt, the king suddenly died, in the year 486 B.C.