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's strategy involved using deception and intimidation to force the Aetolians to abandon Thermopylae, leading to a strategic advantage for the Gauls. The success of this plan was contingent on the cooperation of the locals who provided access to a mountain pass that allowed Brennus to attack the Greeks from behind.