Following the Battle of Gaugamela in 331 BC, in which Alexander the Great defeated Darius III, Darius III began his retreat to Bactria, however he was overthrown by the Satrap of Bactria, Bessus, who continued the retreat eastward through Aria and Margiana. Bessus, who had expected an attack from Alexander along the Silk Road, was surprised when Alexander had advanced through Gedrosia and Arachosia and crossed the Hindu Kush mountains in 329 BC to invade Bactria. Bessus fled north to Sogdia where he too was betrayed and was handed over to Alexander by his courtiers, Spitamenes and Datames.