When Hannibal crossed the Alps in 218 BC, the North African and Spanish forces that he brought with him were outnumbered by Gallic soldiers, who were ferocious fighters and sworn enemies of Rome. For seventeen years, as Hannibal continued to pillage the Italian countryside, and fight pitched battles with the Roman troops, many of his North African and Spanish soldiers died. New waves of Gauls kept joining his crusade against Rome and that pushed the proportion of Gauls in his army higher. Thus, Hannibal’s war on Rome can be seen as another chapter in the centuries old feud between the Gauls and the Romans.
The nomadic Gauls or Celts had conquered much of central Europe by the sixth or fifth century BC. After that they started expanding into Northern Italy and there were frequent fights between them and the Romans. One of these Gallic tribes was called Senones—they settled on the Adriatic coast. According to Livy, the tension between the Senones and the Romans flared up when a Roman aristocrat killed a Senone chieftain during a local dispute. The Senones asked the Romans to surrender the aristocrat who had killed their chieftain. When the Romans refused, the Senones and their Gallic allies declared war on Rome.
The battle between the Gauls and the Romans was fought in 387 BC at the confluence of the Tiber and Allia rivers, Plutarch writes that the Romans were not outnumbered in the battle. Both sides had approximately 40,000 soldiers. Plutarch notes that most Romans were untrained and did not have the experience of using weapons in war.
Livy does not talk about the size of the belligerent forces but he provides lots of information on the social conditions and the actual battle. He writes that the Gauls marched to Rome so quickly that "Rome was thunderstruck by the swiftness at which they moved.” Livy stresses the difference between the Gauls and the Romans. He calls the Gauls: “outlandish warriors who fight naked and are armed with strange weapons,” “strange enemy from the ends of the earth,” “enemies who utter cries like the howling of wolves and barbaric songs,” “the infernal giants.” The Gauls were probably larger and stronger than the Romans.
Concerned about being outflanked by the Gallic army, the commanders on the Roman side created a battle formation that was a thin line extending on both sides, and they stationed a bulk of their troops on top of a nearby hill. When the Senones chieftain saw the Roman reserves on the hill, he thought that they would attack his forces from the rear. Instead of targeting the Roman line in the plains, he led his Gallic troops up the hill to attack the Romans stationed there. After capturing the hill, the Gauls descended on the Roman line.
The Roman army panicked; their formation was shattered at the first impact of the Gauls. The surviving members of the Roman left wing jumped into the Tiber. Weighted down by their armor, many drowned. Those who made it to the other side of the river, escaped to the town of Veii, which the Romans had conquered from the Etruscans in 396 BC.
Another group of Roman soldiers fled to Rome and they made a last stand on the Capitoline Hill, where the women, elderly, and children of Rome had locked themselves in the Citadel. The Gauls marched into Rome unopposed—they were surprised by their easy victory. They sacked the city and besieged the Capitoline Hill. When one group of Gauls tried to rush up the hill, the Romans charged back with ferocity and inflicted heavy casualties on the enemy. The Gauls realized that the Capitoline Hill could not be easily captured. They settled down for a prolonged siege. The area was full of dead bodies which the Gauls had not buried, and an epidemic broke out. Many Gallic soldiers died of disease.
The situation in the Roman camp at the Capitoline Hill was desperate. They entered into negotiation with the Gauls. It was agreed that the Gauls would leave Rome after a payment of one thousand pounds of gold. But the Romans discovered that the weight that the Gauls were using was heavier than the standard weight. When they complained, according to tradition, the Chief of the Senones threw his sword and belt on the scales and shouted, “Vae victis!" ("woe to the conquered”). Now the Romans needed to bring more gold, since they needed to counterbalance the Senones Chief’s sword and belt.
Meanwhile, Marcus Furius Camillus, Roman statesman, had managed to regroup the Roman forces in Veii. He arrived at the area where the gold was being weighed, and throwing his own sword on the scales, he said, “Non auro, sed ferro, recuperanda est patria" ("not with gold, but with iron, will the fatherland be regained”). The appearance of Camillus and his army brought an end to the shameful transaction that was taking place. A battle broke out between the Romans and the Gauls in the streets of Rome. The forces under Camillus inflicted two defeats on the Gauls—first inside the city, and then outside the city. The Gauls were forced to retreat. Later Camillus was feted as the "second Romulus,” the second founder of Rome.
The feud between the Romans and the Gauls went on till the time of Caesar and Augustus. Polybius, the Greek historian of the Hellenistic age, has written that the Romans became militarized because of their long struggle against the Gauls.