The battle on 1 October 331 BCE at which Alexander the Great finally defeated the Persian emperor Darius. Gaugamela was near the city of Arbela, now Irbil in Iraqi Kurdistan, so the Battle of Gaugamela is also known as the Battle of Arbela. Gau-Gamela means camel's grazing place.

Alexander had already defeated the Persians at Granicus on the Aegean coast of Asia Minor in 334, then at Issus in Cilicia in 333. He turned south to conquer Syria, Palestine, and Egypt, then returned to enter Mesopotamia. Darius offered to cede Alexander all his empire west of the Euphrates.

Alexander's friend and adviser Parmenio said, "If I were Alexander, I would accept this;" to which the king replied, "So would I, if I were Parmenio."

He crossed first the Tigris high up, north of Arbela, and descended. Darius' enormous army panicked and was routed; and Darius was killed by one of his satraps a little later. Alexander the Great was proclaimed King of Asia, and moved on to take the cities of Babylon, Susa, and Persepolis.

Arrian, the historiographer of Alexander, said the Persians had about a million men, a front line of chariots, light cavalry at the flanks, and their infantry in the centre. The Macedonians had 40 000 infantry and 7 000 cavalry. Alexander attacked one flank until he could gain access to the centre, when he charged in with his cavalry. The Persians had losses of 300 000, the Macedonians one hundred.