Brennus sent 40,000 infantry and 800 cavalry under Combutis and Orestorius back over the Spercheius to invade Aetolia, hoping to persuade the Aetolian contingent in the Greek army to leave Thermopylae and return to defend their homeland. The plan worked, but the returning Aetolians inflicted such losses on the Gauls that less than half of them returned to Thermopylae. Meanwhile, the locals were intimidated into showing Brennus a mountain pass that would allow him to attack the Greek rear.
Brennus dispatched 40,000 infantry and 800 cavalry under Combutis and Orestorius back across the Spercheius to invade Aetolia, aiming to persuade the Aetolians in the Greek army to abandon Thermopylae and return to defend their homeland. The strategy succeeded, but the returning Aetolians inflicted such heavy casualties on the Gauls that less than half of them returned to Thermopylae.  Meanwhile, local inhabitants were intimidated into revealing a mountain pass that would enable Brennus to attack the rear of the Greeks.