Alexander returned to Phoenicia in the spring of 331. He then directed his march through Samaria, and arrived at Thapsacus on the Euphrates about the end of August. after crossing the river he struck to the north-east through a fertile and well-supplied country. On his march he was told that Darius was posted with an immense force on the left bank of the Tigris; but on arriving at that river he found nobody to dispute his passage. He then proceeded southwards along its banks, and after four days' march fell in with a few squadrons of the enemy's cavalry. From some of these who were made prisoners Alexander learned that Darius was encamped with his host on one of the extensive plains between the Tigris and the mountains of Kurdistan, near a village called Gaugamela (the Camel's House). The town of Arbela, after which the battle that ensued is commonly named, lay at about twenty miles distance, and there Darius had deposited his baggage and treasure. That monarch had been easily persuaded that his former defeat was owing solely to the nature of the ground; and, therefore, he now selected a wide plain for an engagement, where there was abundant room for his multitudinous infantry, and for the evolutions of his horsemen and charioteers. Alexander, after giving his army a few days' rest, set out to meet the enemy soon after midnight, in order that he might come up with them about daybreak. On ascending some sand-hills the whole array of the Persians suddenly burst upon the view of the Macedonians, at the distance of three or four miles. Darius, as usual, occupied the centre, surrounded by his body-guard and chosen troops. In front of the royal position were ranged the war-chariots and elephants, and on either side the Greek mercenaries, to the number, it is said, of 50,000. Alexander spent the first day in surveying the ground and preparing for the attack; he also addressed his troops, pointing out to them that the prize of victory would not be a mere province, but the dominion of all Asia. Yet so great was the tranquillity with which he contemplated the result, that at daybreak on the following morning, when the officers came to receive his final instructions, they found him in a deep slumber. His army, which consisted only of 40,000 foot and 7000 horse, was drawn up in the order which he usually observed, namely, with the phalanx in the centre in six divisions, and the Macedonian cavalry on the right, where Alexander himself took his station. The Persians, fearful of being surprised, had stood under arms the whole night, so that the morning found them exhausted and dispirited. Some of them, however, fought with considerable bravery; but when Alexander had succeeded in breaking their line by an impetuous charge, Darius mounted a fleet horse and took to flight, as at Issus, though the fortune of the day was yet far from having been decided. At length, however, the rout became general. Whilst daylight lasted Alexander pursued the flying enemy as far as the banks of the Lycus, or Greater Zab, where thousands of the Persians perished in the attempt to pass the river. After resting his men a few hours Alexander continued the pursuit at midnight in the hope of overtaking Darius at Arbela. The Persian monarch, however, had continued his flight without stopping; but the whole of the royal baggage and treasure was captured.

Finding any further pursuit of Darius hopeless, Alexander now directed his march towards Babylon. At a little distance from the city the greater part of the population came out to meet him, headed by their priests and magistrates, tendering their submission and bearing with them magnificent presents. Alexander then made his triumphant entry into Babylon, riding in a chariot at the head of his army. The streets were strewed with flowers, incense smoked on either hand on silver altars, and the priests celebrated his entry with hymns. Nor was this a mere display of a compulsory obedience. Under the Persian sway the Chaldaean religion had been oppressed and persecuted; the temple of Belus had been destroyed and still lay in ruins; and both priests and people consequently rejoiced at the downfall of a dynasty from which they had suffered so much wrong. Alexander observed here the same politic conduct which he had adopted in Egypt. He caused the ruined temples to be restored, and proposed to offer personally, but under the direction of the priests, a sacrifice to Belus. Alexander contemplated making Babylon the capital of his future empire. His army was rewarded with a large donative from the Persian treasury; and after being allowed to indulge for some time in the luxury of Babylon, was again put in motion, towards the middle of November, for Susa. It was there that the Persian treasures were chiefly accumulated, and Alexander had despatched one of his generals to take possession of the city immediately after the battle of Arbela. It was surrendered without a blow by the satrap Abulites. The treasure found there amounted to 40,000 talents in gold and silver bullion, and 9000 in gold Darics. But among all these riches the interest of the Greeks must have been excited in a lively manner by the discovery of the spoils carried off from Greece by Xerxes. Among them were the bronze statues of Harmodius and Aristogiton, which Alexander now sent back to Athens, and which were long afterwards preserved in the Ceramicus.