The sack of Rome (390 B.C.) was the worst recorded disaster in the history of the early Roman Republic, and saw a Gallic war band led by Brennus capture and sack most of the city, after winning an easy victory on the Allia.
The speed with which the Gauls had approached the city seems to have caught the Romans by surprise. They were only able to raise a small army, which was easily defeated on the Allia. Most of the survivors took refuge in Veii, which was much better fortified than Rome. The rest escaped back to Rome, where they took refuge in the Citadel, the strongest defensive position in the city.
According to Livy the Gauls reached Rome at about sunset on the day of the battle, but decided not to risk entering an unfamiliar city in the dark. This gave the Romans time to prepare for a siege. A large part of the population escaped across the Tiber to the Janiculum hill and fled to safety, while the Vestal Virgins with their sacred relics escaped to the friendly Etruscan city of Caere. The men of military age and their families took refuge in the Citadel, which could be defended much more easily than the apparently incomplete or weak city walls.
On the following day the Gauls made an unopposed entry into the city. Finding it empty they looted it, and then turned to the Citadel. A direct assault failed, and after that they settled down into a blockade, probably using Rome as a base for raiding Latium while conducting a fairly loose blockade of the Citadel. The Romans were able, with difficulty, to open communications with Veii and eventually a truce was arranged with the defenders, ending active fighting but not the blockade.
Eventually the defenders of the Citadel ran out of food, and were forced to come to terms with the Gauls. A ransom of 1000lbs of gold was agreed. When the Romans complained about the Gaul's scales Brennus is said to have thrown his sword onto the scales with the words 'Woe to the vanquished' (vae victis). After this the Gauls probably left the city safely, taking their plunder with them.
A number of legends developed around the sack of Rome and the siege of the Citadel. Livy reports that as the fighting men took refuge in the Citadel the older senators decided to remain in the Forum to avoid becoming a burden to the defenders, where they were all killed by the Gauls.
The most famous story from the siege involved the sacred geese in the temple of Juno. After the blockade had been going on for some time the defenders decided to appoint Furius Camillus as dictator. To do this a messenger had to leave the city, and the Gauls noticed the route he took to climb down the cliffs that defended the Citadel. On the next suitable night the Gauls climbed up this route, up to the temple of Carmentis. Neither the dogs nor the guards heard them, but the geese did, and the noise they made alerted M. Manlius, a former consul, who fought off the first Gauls and gave the rest of the defenders time to come to the danger point.
Furius Camillus featured more directly in another probable legend. In this version of events he was able to raise an army from his position in exile, and arrived at Rome just in time to prevent the ransom from being paid. When the Gauls attacked Camillus's army they were defeated, once in the city and once at the eighth milestone on the road to Gabii, where their army was annihilated. This story was probably based on the battle of the Trausian Plain, in which an Etruscan army from the city of Caere defeated part of the Gallic force and recovered some of the gold taken from Rome, and was later modified to preserve Roman pride.
The sack of Rome does not seem to have been as disastrous as the Roman tradition claimed. No archaeological evidence has been found to support the idea that the city was burnt down, and Roman power seems to have recovered very quickly after the disaster. Early evidence suggests that the city was at least partly evacuated before the Gauls arrived - according to the Roman tradition the Vestal Virgins and their sacred objects were escorted to Caere by Lucius Albinius (possibly the same Lucius who Aristotle credited with saving the city). Brennus and his band appear to have been on their way to the south of Italy, where they signed on as mercenaries serving Dionysius of Syracuse. If this is the case then they would have only been interested in easily movable plunder - including the gold they were paid to leave the city. Combined with the low losses probably suffered in the brief battle on the Allia this suggests that although the Gallic raid was humiliating, it was not particularly damaging, although if the Gauls had appeared before the end of the long siege of Veii then they might have caused a real disaster.