The Battle of the Allia, a battle of the first Gallic invasion of Italy, is fought near the Allia river: the defeat of the Roman army opens the route for the Gauls to sack Rome.

The remnants of the legions flee back to Rome in panic; as Livy states, "all hastened to Rome and took refuge in the Capitol without closing the gates."

In Rome the citizens barricade themselves on the Capitoline Hill.

The Gauls unsuccessfully try a full frontal assault, pausing half way up the hill and due to the Romans' quick thinking, and a direct charge later, they pay dearly and lose many lives.

At this point, the Roman soldiers in Veii need to get a message to the Senate in order to reinstate Marcus Furius Camillus as the dictator and general, and so a messenger climbs a steep cliff the Gauls have neglected to guard.

The messenger leaves with the Senate's approval, but the Gauls notice this path onto the hill.

According to legend, Marcus Manlius Capitolinus is alerted to the Gallic attack by the sacred geese of Juno.

The rest of the city is plundered and almost all Roman records are destroyed.

As a result, all Roman history prior to this date is perhaps more legend than fact.

Marcus Furius Camillus may have arrived with a relief army, but this may be Roman propaganda to help quell the humiliation of defeat.

The Gauls may have been ill-prepared for the siege, as an epidemic breaks out among them as a result of not burying the dead.

Brennus and the Romans negotiate an end to the siege when the Romans agree to pay one thousand pounds in gold.