Battle of Gaugamela
- Macedonians and other Greeks under Alexander, 7,250 cavalry and 40,000 infantry. (According to Arrian)
- Persians under Darius, with maybe 35,000 mostly heavy cavalry, 200,000 infantry (including some 10,000 Greek mercenaries). The actual Persian numbers are unknown, although it is fairly clear that they substantially outnumbered Alexander's forces.
Darius chose (or smoothed out, depending on accounts) a flat plain where he could deploy his numerically superior cavalry forces. The location of the battle - near Tel Gomel, east of Mosul in northern modern-day Iraq - was determined by Sir Aurel Stein in 1938 (see his Limes Report, pp. 127-1). After the battle, Darius fled to Arbela (modern-day Arbil) 120 km to the east. The distance from Arbil to Mosul is roughly 80 km, so considerable room for discussion remains.
During the two years after the Battle of Issus Alexander proceeded to occupy the Mediterranean coast and Egypt. He then advanced from Syria against the heart of the Persian empire. Alexander crossed both the Euphrates and the Tigris rivers without any opposition.
The battle began with the Persians already present at the battlefield.
Darius had recruited the finest cavalry from his satrapies and from an allied Scythian tribe. Darius also deployed scythed chariots for which he had prepared cleared terrain in front of his troops. He also had 50 Indian elephants supported by Indian chariots, although these seemingly played no role in the battle.
Before the battle, Darius ordered bushes and vegetation removed from the battlefield, to maximize the chariots' effectiveness.
Darius placed himself in the center with his best infantry as was the tradition among Persian Kings. He was surrounded by, on his right, the Carian Cavalry, Greek mercenaries, and the Persian Horse Guards. In the right-center he placed the Persian Foot Guards (Apple Bearers/Immortals to the Greeks), the Indian Cavalry and his Mardian archers.
On both flanks were the cavalry. Bessus commanded the left flank with the Bactrians, Dahae Cavalry, Arachrosian Cavalry, Persian Cavalry, Susian Cavalry, Cadusian Cavalry, and Scythians. Chariots were placed in front with a small group of Bactrians.
Mazaeus commanded the right flank with the Syrian, Median, Mesopotamian, Parthian, Sacian, Tapurian, Hyrcanian, Albanian, Sacesinian, Cappadocian, and Armenian cavalry. The Cappadocians and Armenians were stationed in front of the other cavalry units, and led the attack. The Albanian and Sacesinian cavalry were sent around to flank the Macedonian left.
The Macedonians were divided into two, with the right side of the army falling under the direct command of Alexander, and the left to Parmenion. Alexander fought with his Companion Cavalry. With it were the Paeonian, and Macedonian light cavalry. The mercenary cavalry was divided into two groups, with the veterans being stationed on the flank of the right, and the rest being put in front of the Agrians and Macedonian archers which were stationed next to the phalanx.
Parmenion was stationed on the left with the Thessalian, Greek mercenary, and Thracian cavalry units. There they were to pull off a holding maneuver while Alexander landed the decisive blow from the right.
On the right-center of the formation Cretan mercenaries. Behind them was a group of Thessalian cavalry under Phillip, and Achaian mercenaries. To their right was another part of the allied Greek cavalry. From there came the phalanx, which was placed into a double-line. Due to being outnumbered over 5:1 in cavalry, and their line surpassed by over a mile, it seem inevitable that the Macedonians would be flanked by the Persians. The second line were given orders to deal with any flanking units should the situation arise. This second line consisted of mostly mercenaries.
During the battle Alexander used a unique strategy which has been duplicated only a few times throughout history. His plan was to draw as much of the Persian cavalry as possible to the flanks. The purpose of this was to create a gap within the enemy line where a decisive blow could then be struck at Darius in the center. This required almost perfect timing and maneuvering, and the Great King himself to act first. The Macedonians advanced with the wings echeloned back at 45 degree angles to lure the Persian cavalry to attack. At the same time they slowly moved to the right. Alexander forced Darius to attack (as they would soon move off the prepared ground) though Darius did not want to be the first to attack after seeing what happened at Issus against a similar formation. In the end Darius's hand was forced, and he attacked.
Darius now launched his chariots, some of which were intercepted by the Agrianians. It is said that the Macedonian army had trained for a new tactic to counter these devastating chariots if they ran into their ranks. The first lines would step aside, opening a gap. The horse would refuse to run into the lances of the front ranks, and enter the "mouse trap", only to be stopped by the lances of the rear ranks. The chariotteers could then be killed at leisure. The chariots were rendered useless.
As the Persians moved farther and farther to the Macedonian flanks in their attack, Alexander slowly filtered in his rear-guard. The Persians followed suit, until finally a gap opened between Bessus's left and Darius's center as Alexander had sent in his last mounted reserves against the Persians. Alexander disengaged his Companions, and prepared for the decisive attack on the Persians. Leading the way, he formed his units into a giant wedge, with him leading the charge. Behind them guards brigade, and any phalanx battalions he could withdraw from the battle. These were followed up light troops.
This large wedge then smashed right into the weakened Persian center, taking out Darius's royal guard, and the Greek mercenaries. Bessus on the left, now cut off from Darius, and fearing he himself would be struck with this wedge, began to pull back his forces. Darius was in danger of himself being cut off, and here several accounts differ as to what happened. According to most accounts, Darius broke and ran, and the rest of his army followed suit. However the only preserved contemporary account, an astronomical diary from Babylon written within days of the battle, says this:
- The twenty-fourth [day of the lunar month], in the morning, the king of the world [i.e., Alexander] [erected his] standard [lacuna]. Opposite each other they fought and a heavy defeat of the troops [of the king he inflicted]. The king [i.e., Darius], his troops deserted him and to their cities [they went]. They fled to the land of the Guti. 
Diodorus agrees with the story, backing up its validity. It would seem this is the most likely account of the battle.
Alexander could not pursue Darius at this point, however. He received desperate messages from Parmenion (an event which would later be used by Callisthenes and others to discredit Parmenion) on the left.
While holding on the left, a gap had also opened up between the left and center of the Macedonian line. The Persian and Indian cavalry units stationed in the center with Darius broke through. Instead of taking the phalanx or Parmenion in the rear, however, they continued on towards the camp to loot. On their way back, they would run into Alexander and the Companions, which led to deaths of over 60 Companions.
Meanwhile, as the center and Darius broke, Mazaeus also began to pull his forces back as Bessus had. However, unlike on the left with Bessus, the Persians soon fell into disorder as the Thessalians and other cavalry units charged forward at their fleeing enemy.
After the battle, Parmenion rounded up the Persian baggage train while Alexander and his own bodyguard chased after Darius in hopes of catching up. As at Issus, substantial amounts of loot were gained following the battle. 4,000 talents were captued, as well as the King's personal chariot and bow. The war elephants had also been rounded up.
Darius had managed to escape the battle with a small core of his forces remaining in tact. The Bactrian cavalry and Bessus managed to catch up with him, as well as some of the survivors of the Royal Guard and 2,000 Greek mercenaries.
At this point, the Persian Empire was divided into two halves - East and West. Alexander would go on to proclaim himself Great King. On his escape, Darius gave a speech to what remained of his army. He planned to head further East, and raise another army to face Alexander while he and the Macedonians head to Babylon. At the same time he dispatched letters to his Eastern satrapies asking them to remain loyal.
- Alexander the Great: An annotated list of primary sources from Livius.org
- Wiki Classical Dictionary, extant sources and fragmentary and lost sources
- Plutarch, Life of Alexander (in English)
- Justin, Epitome of the Philippic History of Pompeius Trogus (in English)
- Plutarch, Of the Fortune or Virtue of Alexander the Great (in English)
- Quintus Curtius Rufus, Histories of Alexander (in Latin)
- Alexander of Macedon 356-323 B.C. by Peter Green
- Alexander by Theodore Ayrault Dodge
- J.F.C. Fuller. A Military History of the Western World. Three Volumes. New York: Da Capo Press, Inc., 1987 and 1988.
- v. 1. From the earliest times to the Battle of Lepanto; ISBN 0306803046: pp. 87 to 114 (Alexander the Great).
- De Santis, Marc G. “At The Crossroads of Conquest.” Military Heritage. December 2001. Volume 3, No. 3: 46-55, 97 (Alexander the Great, his military, his strategy at the Battle of Gaugamela and his defeat of Darius making Alexander the King of Kings).
- Livius.org tells the story of Alexander and quotes original sources. Favors a reconstruction of the battle which heavily privileges the Babylonian astronomical diaries.