It was now almost two years since the battle of Issus, and Alexander determined once more to meet Darius, who had again assembled a large army.
In the spring of 331 b.c. the king went back to Tyre, and by August he had reached Thapsacus, a town on the banks of the river Euphrates. He wished to go on to Babylon, the capital of the Persian empire, but the direct way to the city, which was down the Euphrates, was guarded by Cyrus with a large army. So Alexander struck off across the north of Mesopotamia, and reaching the Tigris marched along the river on the eastern side. Above Nineveh he crossed to the other bank, and after marching southward for several days, he heard that Darius was encamped on a plain near Gaugamela, on the river Bumōdus.
Even to the brave Macedonian generals, the vast hosts of the Persians looked formidable.
Parmenio looking at them begged the king to surprise the enemy by a night attack rather than risk a battle in daylight.
"I will not steal a victory," answered Alexander.
The night before the battle the king slept soundly, as though nothing preyed upon his mind. In the morning his generals found him still fast asleep, so without disturbing him they themselves bade the soldiers have breakfast.
At length Parmenio went to wake the king, and having with difficulty roused him, he asked how it was possible he could sleep so soundly when the most important battle of his life had to be fought that day.
"You slept, sire, as though you were already victorious," said the anxious general.
"Are we not so indeed," answered the king, "since we are at last relieved from the trouble of wandering in pursuit of Darius, through a wide and wasted country, hoping in vain that he would fight us?"
Alexander, who was already dressed, now put on his helmet, which was of iron, yet so polished was it that it shone as silver. Great skill had been lavished on the decoration of his belt, which was indeed the most splendid part of his dress. He then ordered his army to be drawn up in battle array, while he mounted Bucephalus, who was old now, yet eager for battle.
Before the king gave the signal to attack, he stretched out his right hand to heaven, and called upon the gods to defend and strengthen the Greeks, if he indeed were the son of Zeus.
By the side of Alexander rode a soothsayer, clad in a white robe and wearing on his head a crown of gold. He pointed to the sky, and the soldiers looking up saw an eagle flying over the king's head and on toward the Persian army. "It is a good omen," they cried, and shouted to be led at once against the foe.
A moment later the order was given, and the Macedonians rushed upon the great hosts of the enemy.
Darius thought that his war-chariots would cause deadly havoc among his enemies, for scythes were fastened to the wheels to mow down all who came within reach.
But the Macedonian archers drew their bows and sped their arrows among the charioteers, while the strongest seized the reins of the horses, and pulled the drivers from their seats. Then the soldiers opened wide their ranks so that those chariots that still had drivers rattled harmlessly past them.
Alexander was already attacking the centre of the Persian army, where, as at the battle of Issus, Darius sat in his chariot, looking on at the struggle.
All at once he saw Alexander with his chosen companions drawing nearer and nearer, and once again his courage failed. Fiercer and fiercer raged the battle, closer and closer drew Alexander to the Persian king.
The horsemen grouped in front of Darius were driven backward and fled, all save the bravest who never flinched, but fell in a supreme effort to keep the enemy from approaching any nearer to the king's chariot.
Even as they fell they still tried to keep back the foe, clinging desperately to the legs of the horses as they galloped over their wounded bodies.
Darius was in immediate danger of being captured. In vain the driver tried to turn the royal chariot, the bodies of the fallen soldiers would not allow the wheels to move. The horses plunged and kicked in an agony of fear, and the charioteer was helpless.
Then, as the king had done on the field of Issus, he did now. He leaped from the chariot, mounted a horse and fled from the battlefield.
Alexander followed the king in swift pursuit; it seemed impossible that he could escape. But Parmenio, who was commanding the left wing, was almost overpowered by the enemy. He sent a messenger to overtake Alexander, and beg him for help.
The king reluctantly gave up his pursuit of Darius, and rode back with his companions to give his general the help he had entreated. But by the time he reached the left wing his aid was no longer needed. Parmenio had wrestled victory from the foe.
So the king again set out in pursuit of Darius, but all that he captured was the chariot, the shield and the bow of the coward king.