He then again set out to meet Darius; in October, 331 B.C., a great battle was fought on the plain stretching eastward to Arbela. Notwithstanding the immense superiority of his adversary, who had collected a new army of more than a million men, Alexander was not for a moment doubtful of victory. Heading the cavalry himself, he rushed on the Persians, and put them to flight; then hastened to the assistance of his left wing, which, in the meanwhile, had been sorely pressed. He was anxious to make Darius a prisoner, but Darius escaped on horseback, leaving his baggage and all his treasures a prey to the conqueror. Babylon and Susa, the treasure-houses of the East, opened their gates to Alexander, who next marched toward Persepolis, the capital of Persia, which he entered in triumph.
The marvellous successes of Alexander now began to dazzle his judgment and to inflame his passions. He became a slave to debauchery, and his caprices were as cruel as they were ungrateful. In a fit of drunkenness, and at the instigation of Thaïs, an Athenian courtesan, he set fire to Persepolis, the wonder of the world, and reduced it to a heap of ashes; then, ashamed of the deed, he set out with his cavalry in pursuit of Darius. Learning that Bessus, the Bactrian satrap, held him a prisoner, he hastened his march, in the hope of saving him, but he found him mortally wounded (330 B.C.). He mourned over his fallen enemy, and caused him to be buried with all the customary honors, while he hunted down Bessus, who himself aspired to the throne, chasing him over the Oxus to Sogdiana (Bokhara).
Having discovered a conspiracy in which the son of Parmenio was implicated, he put both father and son to death, though Parmenio himself was innocent of any knowledge of the affair. This cruel injustice excited universal displeasure. In 329 he penetrated to the farthest known limits of Northern Asia, and overthrew the Scythians on the banks of the Jaxartes. In the following year he subdued the whole of Sogdiana, and married Roxana, whom he had taken prisoner. She was the daughter of Oxyartes, one of the enemy's captains, and was said to be the fairest of all the virgins of Asia. The murder of his foster-brother, Clitus, in a drunken brawl, was followed, in 327 B.C., by the discovery of a fresh conspiracy, in which Callisthenes, a nephew of Aristotle, was falsely implicated. For challenging Alexander's divinity, he was cruelly tortured and hanged.