In 334 BCE, Alexander the Great of Macedon reinforced his already mighty army, marched into Asia Minor, and began his great campaign to conquer the Persian Empire. Within two years he held half that empire, had eliminated the Persian fleet's Mediterranean bases, and cheekily seized Egypt.
As Alexander (356-323 BCE) advanced into Mesopotamia in 331 BCE, Persia's King Darius (d.330 BCE) considered retreating further into his territory and perhaps scorching the earth behind him, but instead decided to give battle.
Darius chose his ground near the modern city of Mosul in Iraq. Darius' initial attack would be made by his first line, which was composed mostly of cavalry with a few of the best Persian infantry units mixed in. Darius himself was in the Persian centre with the cavalry and infantry of his personal guard. In front of them were arrayed some 200 scythed chariots. A second line, composed of enormous numbers of infantry, was positioned behind the first. Darius expected to be able to envelop both of Alexander's flanks with his vastly superior numbers and to crush his army from all sides.
The Battle Commences
To help protect his flanks from envelopment, Alexander positioned his cavalry on the flanks, with each unit echeloned back from the last, creating 'refused' flanks that required the enemy to move further if he wanted to engage them. His phalanx of well-drilled Macedonian infantry was positioned in the centre of the Greek line. A reserve phalanx formed Alexander's second line.
Alexander's plan called for his left flank, under the veteran general Parmenio, to fight a holding action while Alexander led the right-wing to victory. He was assisted in this by the Persians themselves, who sent their left-flank cavalry far around Alexander's right, opening up a gap in their line. The Macedonian army was drifting to the right. If Darius delayed too long his chariots might not gain the benefits of their prepared run.
So the attack was launched. Darius' cavalry swept around the flanks of the Macedonian force as the chariots (and, in some accounts, 15 war elephants) made their initial frontal attack. The Macedonian line was able to fend off the chariots and weather the initial onslaught, though some of the Persian cavalries were able to break through. They were engaged by the reserve phalanx and by light troops, which had been briefed for the task.
As he had planned, Alexander now lead his elite Companion Cavalry and other units against the Persian left. Exploiting the gap that was opening between the Persian cavalry on the left and the centre of their army, the Companions inflicted a savage blow on the forces in front of them. They were followed by a great wedge of infantry and light troops which fell on the disorganized Persians. Fearing that he would be cut off, the commander of the Persian left wing, Bessus, began to retire, which eased the pressure on Alexander's refused right flank. Meanwhile, Darius himself was feeling the pressure and began to retreat. The withdrawal became a route as Darius' bodyguards followed their leader in fleeing the field.
Alexander was not able to pursue, as the situation on his left was becoming desperate. Parmenio's forces were hard-pressed on the left but had done their job, tying down the Persians' forces. Now Alexander fell upon the rear of the Persians engaging Parmenio, forcing them to retreat. The Persian right-wing commander, Mazaeus, tried to conduct an orderly retreat but was vigorously attacked and his troops thrown into confusion. This moment represented the end of organized Persian resistance at Gaugamela. Behind the fleeing Darius was left 40,000 dead Persian troops, 4,000 more as prisoners and the remainder of the army scattered around the countryside. Battle Of The Granicus: Alexander The Great Defeats Persian Cavalary Force On His Way To Asia Minor