(Oct. 1, 331 BC) clash between the forces of Alexander the Great of Macedonia and Darius III of Persia that decided the fate of the Persian empire. Attempting to stop Alexander's incursion into the Persian empire, Darius prepared a battleground on the Plain of Gaugamela, near Arbela (present-day Irbīl in northern Iraq), and posted his troops to await Alexander's advance. Darius had the terrain of the prospective battlefield smoothed level so that his many chariots could operate with maximum effectiveness against the Macedonians. His total forces greatly outnumbered those of Alexander, whose forces amounted to about 40,000 infantry and 7,000 cavalry.

Alexander's well-trained army faced Darius' massive battle line and organized for attack, charging the left of the Persians' line with archers, javelin throwers, and cavalry, while defending against Darius' outflanking cavalry with reserve flank guards. A charge by Persian scythed chariots aimed at the centre of Alexander's forces was defeated by Macedonian lightly armed soldiers. During the combat, so much of Darius' cavalry on his left flank were drawn into the battle that they left the Persian infantry in the centre of the battle line exposed. Alexander and his personal cavalry immediately wheeled half left and penetrated this gap and then wheeled again to attack the Persians' flank and rear. At this Darius took flight, and panic spread through his entire army, which began a headlong retreat while being cut down by the pursuing Greeks. The Macedonian victory spelled the end of the Persian empire founded by Cyrus II the Great and left Alexander the master of southwest Asia.