Cyrus was succeeded by his son Cambyses, who though not devoid of fine qualities was jealous and tyrannical. He caused his own brother Smerdis to be put to death. He completed the conquests of his father by adding Egypt to his empire. In a fit of remorse for the murder of his brother he committed suicide, and the empire was usurped by a Magian impostor, called Gaumata, who claimed to be the second son of Cyrus. His reign, however, was short, he being slain by Darius the son of Hystaspes, belonging to another branch of the royal family. Darius was a great general and statesman, who reorganized the empire and raised it to the zenith of its power and glory. It extended from the Greek islands on the west to India on the east. This monarch even penetrated to the Danube with his armies, but made no permanent conquest in Europe. He made Susa his chief capital, and also built Persepolis, the ruins of which attest its ancient magnificence. It seems that he was a devout follower of Zoroaster, and ascribed his successes to the favor of Ahura-Mazda, the Supreme Deity.
It was during the reign of Darius that Persia came in contact with Greece, in consequence of the revolt of the Ionian cities of Asia Minor, which, however, was easily suppressed by the Persian satrap. Then followed two invasions of Greece itself by the Persians under the generals of Darius, and their defeat at Marathon by Miltiades.
Darius was succeeded by Xerxes, the Ahasuerus of the Hebrew Scriptures, whose invasion of Greece with the largest army the world ever saw properly belongs to Grecian history. It was reserved for the heroes of Plataea to teach the world the lesson that the strength of armies is not in multitudes but in discipline,--a lesson confirmed by the conquests of Alexander and Caesar.
On the fall of the Persian Empire three hundred years after the fall of Babylon, and the establishment of the Greek rule in Asia under the generals of Alexander, Persia proper did not cease to be formidable. Under the Sassanian princes the ambition of the Achaemenians was revived. Sapor defied Rome herself, and dragged the Emperor Valerian in disgraceful captivity to Ctesiphon, his capital. Sapor II. was the conqueror of the Emperor Julian, and Chrosroes was an equally formidable adversary. In the year 617 A.D. Persian warriors advanced to the walls of Constantinople, and drove the Emperor Heraclius to despair.