In the 5th and early 4th centuries BC, migratory Germanic tribes pressured Gallic Celts living in the Danube regions to push South in search of new territory. They were likely familiar with the Po River Valley, in north central Italy, from trade arrangements with Etruscans who were there. The Gauls crossed the Alps en masse capturing and settling Etruscan territory by force. The Gallic tribes were united only by blood and origin and each maintained their own kings or warlords. Some of these tribes settled into cattle and cereal farming along with peaceful cohabitation, but others maintained aggressive policies towards their new neighbors.
One such tribe, the Senones, was under the command of a Brennus, who led his Celts to the Etruscan city of Clusium about 100 miles north of Rome. It is important to note that much of the ancient source material, such as Livy, Polybius and Diodorus Siculus, is steeped in legend or, especially on the part of Livy, biased though nationalism. Whether Clusium was the target, or it simply stood in the path on the way to the more powerful city of Rome, is unclear. It is clear, however, that the Celts did approach and lay siege to Clusium and that the Etruscans there likely set aside any differences and called to Rome for help.
In response, according to the ancients, the Romans sent a delegation of 3 envoys to treat with Brennus. Siculus claims that the 3 were really spies sent to assess the strength of the Celts, but it is apparent that whatever the reason for the meeting, it escalated into violence. After exchanged insults, the Roman envoys were involved in a skirmish with the Gauls, in which one Celtic chief tan was killed. The commissioners returned to Rome without relief for Clusium and with an angry Gallic army behind them. Brennus sent his own representatives to Rome to demand the 3 men be turned over to him, but was predictably refused. Later that year, the angered Gauls left Clusium behind and headed for Rome to seek revenge.
The advancing Gauls invaded Roman territory and threatened the security of Rome herself. Eleven miles to the north of Rome, an outnumbered Roman army mustered under the command of A.Quintus Sulpicius, met them in July, 387 BC (the traditional date is recorded as 390 BC but the Varronian chronology is erroneous), and suffered a crushing defeat on the banks of the River Allia. As all appeared lost, some Roman defenders retreated to the Capitoline Hill to endure a siege, while civilians fled through the city gates to the city of Veii and the surrounding countryside. The Gauls poured into Rome slaughtering civilians while looting and burning everything in their path. At some point they apparently attempted an uphill attack on the heavily fortified capital, but were repulsed and never able to dislodge the occupants.
For seven months the Gauls remained and wreaked havoc around Rome. Several assaults on the Capitol all failed, and one such night attempt was even said to have been thwarted through the timely intervention of the sacred Geese of the Temple of Juno. In any event, by this point, the Roman garrison must've been getting dangerously low on supplies. The Romans engaged with Brennus for terms that would ensure that the Celts depart and Brennus apparently agreed to leave Rome for the price of 1,000 lbs. of gold. There are theories that the Celts were paying heavy tolls from disease, or that there own settlements to the north were under attack by other Italian tribes. Whatever the reason, Brennus accepted the terms and agreed to leave. The following passage from Livy, regarding these terms, leaves us with one of the most famous lines accredited to a barbarian chief in dealings with Rome.
"Quintus Sulpicius conferred with the Gallic chieftain Brennus and together they agreed upon the price, one thousand pounds' weight of gold. Insult was added to what was already sufficiently disgraceful, for the weights which the Gauls brought for weighing the metal were heavier than standard, and when the Roman commander objected the insolent barbarian flung his sword into the scale, saying 'Vae Victis-- 'Woe to the vanquished!"
With the departure of Brennus and his Gauls, many Romans wanted to abandon their city and move to the nearby city of Veii, but reverence for the gods and the divine will of Roma alleviated this concern. The Romans obviously decided to stay, and quickly rebuilt the city. One major improvement was the completion of the Servian Wall, supposedly built by the Etruscan King Servius Tullus. As a further result of the Gallic invasion, the Romans adopted new military weaponry, abandoning the Greek Phalanx style spears in favor of the gladius and modified armor. Through the resulting civil strife, the legion was reorganized, placing the youngest and strongest soldiers in the front lines, as opposed to the previous formation of order according to wealth.
The Gallic invasion left Rome weakened and also encouraged several previously subdued Italian tribes to rebel. The Etruscans, Volsci, Hernici, and Aequi were all among these numbers. One by one, over the course of the next 50 years, these tribes were defeated and brought back under Roman dominion. Meanwhile, the Celts would continue to harass the region until 345 BC, when they entered into a formal treaty with Rome. Like most others, this treaty would be short lived and the Romans and Celts would maintain an adversarial relationship for the next several centuries. The Celts would remain a threat in Italy until the final defeat of Hannibal in the 2nd Punic War. The sack of Rome would be long remembered by Romans, and would finally be avenged 3 1/2 centuries later with Caesar's conquest of Gaul.