Brennus sacking Rome. No question. Especially since the Roman army at the Battle of the Allia apparently outnumbered the Gauls (Senones) two to one, yet were still badly defeated when the Gauls charged and routed the Roman cavalry and light infantry on the wings, then surrounded the tightly packed Roman phalanx and cut it to pieces. The Romans began moving away from the Greek-style classical hoplite phalanx from this time onward. Soon, only the Triarii remained as hoplites.