Battle of Gaugamela
Leaving Egypt in 331 BC, Alexander marched eastward into Achaemenid Assyria in Upper Mesopotamia (now northern Iraq) and defeated Darius again at the Battle of Gaugamela. Darius once more fled the field, and Alexander chased him as far as Arbela. Gaugamela would be the final and decisive encounter between the two. Darius fled over the mountains to Ecbatana (modern Hamadan) while Alexander captured Babylon.