sometimes called the Battle of Arbela, engagement between the armies of Alexander the Great and the Persian monarch Darius III, fought on Oct. 1, 331 bc. The Persian defeat in the battle signaled the decline of their empire and the opening of the East to the Macedonians.

Marching north from Egypt early in 331 bc, Alexander invaded the Persian Empire for the second time. Darius assembled an army of about 250,000 men on the plain of Gaugamela, some 97 km (about 60 mi) from Arbela (now Irbil, Iraq). The Persians formed a front line of chariots, supported by archers and cavalry. The infantry was massed behind the chariots, and light cavalry guarded the flanks. Alexander, with 40,000 infantry and 7000 cavalry, moved his forces near the Persian position in late September. On October 1, Alexander began the battle by attacking the Persian left flank. Little progress was made until the Persian cavalry became fully engaged on the left flank, leaving the infantry exposed. Alexander then led a charge of his best cavalry into the enemy’s center, broke it, and attacked the Persian flanks and rear. Darius fled, and the Persian army began to retreat in disorder. The Persians were chased for about 80 km (50 mi) by Alexander’s men, who killed large numbers of the fleeing Persians. The Persians lost 40,000 to 90,000 men in the battle; the Macedonians lost fewer than 500.