Note on sources: All existing primary sources on the Battle of Gaugamela are from Greek authors - there are no accounts from the Persian point of view. Most of these "primary" sources also were written long after the fact. As the old saying goes, the victors get to write history, but modern scholars are trying to reconstruct a less biased version of what happened at Gaugamela in 331 BCE.
Two armies faced off across a flat, treeless plain. Darius III, King of the Achaemenid Empire, had chosen this battle site with care. His infantry, his scythed chariots, and his 15 war elephants stood at the ready; the enemy had a much smaller force.
By rights, Darius should have won this battle, the Battle of Gaugamela... but his opponent, Alexander the Great of Macedon, was one of the greatest tacticians the world has ever known.
Background to Gaugamela: Two Kings
Alexander took control of the Hellenic League after his father's assassination in 336 BCE; the new king was only twenty years old. His father, Philip II of Macedon, had unified Greece; Philip and the philosopher Aristotle, who served as Alexander's tutor, taught the young man creative thinking and martial theory.
Whether because of some pan-Greek nationalist sentiment, or simply because some men have a need to conquer the world, Alexander set out to humble the Persian Achaemenid Empire. Under Darius the Great and his son, Xerxes the Great, the Persians invaded and subjugated much of Greece during the Greco-Persian Wars (499-450 BCE). More than a century later, that memory still rankled with the Macedonian kings.
Darius III took his throne-name from the great Persian conqueror of Asia Minor, but his actual blood relationship was likely very tenuous. According to his official genealogy, Darius III's was the great-grandson of Darius II through his father's line, and was also related to the royal family on his mother's side. However, his predecessor, Artaxerxes III, had all of his own brothers, uncles, nephews and male cousins killed to eliminate potential rivals for the throne. Thus, it is unlikely that Darius III was really a cousin of Artaxerxes.
Modern scholars such as Ernst Badian believe that Darius III was originally a minor nobleman and military officer named Codomannus, and that he was a distant relative of the royal family of mixed Achaemenid and Babylonian (Iraqi) descent. He gained promotions thanks to his courage in battle and a fortunate marriage to a Persian noblewoman, and stepped into the kingship after nearly all of the other potential candidates were assassinated.
Darius and Alexander first clashed at the Battle at Issus, in November of 333 BCE. This bloody battle in southern Anatolia (modern-day Turkey) saw Alexander pin the numerically superior Persian army into a narrow coastal plain. Darius was unable to extract his troops, and suffered heavy losses. He retreated to fight another day; Greek sources accused him of cowardice, but as the last surviving adult male with any claim to the Persian throne, his death would have meant the end of the Achaemenid Empire.
During the following two years, Darius recruited a large number of men from the eastern reaches of his empire and trained them as infantry soldiers. He also hired mercenaries from both Greece and India; the Indian troops evidently brought 15 war-elephants, as well. The total number of men in the Persian army is not known; estimates range from about 50,000 to the wildly inflated 1 million cited by some Greek historians.
Meanwhile, Alexander and the Greek army conquered their way around the eastern end of the Mediterranean Sea, even taking Egypt. They then advanced through Babylonia to strike at Persia.
The two armies met in what is today the Kurdistan region of northern Iraq. The Battle of Gaugamela is sometimes called the Battle of Arbela, and many scholars believe that it occurred near the modern-day Kurdish city of Erbil (Arbela). However, the exact battle site is unknown.
The Battle of Gaugamela
To avoid being pinned down again, like at Issus, Darius chose to meet the Macedonians on a flat, treeless plain. Alexander's forces marched to within hailing distance, but did not attack. All through the night of September 30, 331 BCE, the Persians remained on high alert, expecting the Macedonians to launch their assault under cover of darkness. Instead, Alexander allowed his army to sleep so that, unlike their opponents, they would be well-rested.
Early the next morning, October 1, Alexander led the right wing of his formation out toward the Persians in order to draw their attack. Darius seized the bait, sending his Bactrian cavalry crashing into the right flank of the Macedonian formation. Simultaneously, the Persian chariots with scythes on their wheels charged the center of the Macedonian front line.
Alexander had arrayed his troops in three lines, with two rows of cavalry in front, and a third line of archers and javelin-throwers. At both ends of the line were triangular phalanxes of men, jutting forward like canine teeth. The Macedonian soldiers wore iron breast-plates, crested battle helmets, and short tunics. Alexander probably had around 40,000 men, while Darius's army likely was somewhat larger. Reasonable estimates of Persian strength range between about 25,000 and 100,000, with 50,000 a common guess. Greek sources have surprisingly little to say about how the enemy army was arrayed. Persian soldiers of that time wore chain mail armor, and the officers had quilted cloth armor under their mail.
The Persian scythe-chariots rolled into the Greek lines, blades whirling, but Alexander's front-line infantry simply stepped aside. The javelin-throwers lined up behind made short work of Darius' charioteers and their horses. Alexander sent a cavalry counter-charge against the Persians on the right, but with only 400 riders they were soon driven back. The Persian cavalry pressed their advantage, fighting towards Alexander's position. The Macedonian king sent a second, much larger counter-attack of infantry, which drove back the Persians.