Darius III was the Persian king defeated by Alexander the Great at the Battle of Gaugamela in 331 BCE.
The battle was fought near the ancient Assyrian city of Nineveh, probably on a plain prepared by Darius’s forces. Alexander’s Macedonian army was heavily outnumbered according to ancient accounts, though those figures are debated. Alexander used coordinated infantry and cavalry maneuvers to break the Persian center and force Darius to flee.
Gaugamela is often treated as the decisive defeat of the Achaemenid Empire. Darius escaped and attempted to regroup, but he was later murdered in 330 BCE by the satrap Bessus. Alexander subsequently occupied Babylon, Susa, and Persepolis, bringing the Achaemenid royal dynasty to an end.
The battle is sometimes called Arbela, after a nearby city. Gaugamela is the more precise name for the battlefield, while Arbela was an important regional location associated with the campaign.