Battle of Gaugamela
At the Battle of Gaugamela in 331 BC between the forces of Darius III and those of Alexander the Great, one such Parthian unit was commanded by Phrataphernes, who was at the time Achaemenid governor of Parthia. Following the defeat of Darius III, Phrataphernes surrendered his governorate to Alexander when the Macedonian arrived there in the summer of 330 BC. Phrataphernes was reappointed governor by Alexander.