The Gauls Sack Rome and Rome's Allies in Italy Revolt (390 - 338 B. C.)
During the Fifth and early Fourth Centuries B. C., pressure from Germanic tribes to the North of them caused Gallic Celts living in the Danube Regions to migrate South in search of new territory. They crossed the Alps and settled in the Po River valley, conquering the Etruscans living there in about 400 B. C. These Celts earned their livelihood primarily by herding cattle and by warring with their neighbors. They were divided into several tribes, each under the leadership of a petty king or chief warlord. They placed a high value on individual bravery in combat and were known to be fierce warriors.
In 390 B. C., they invaded Roman territory and threatened the security of Rome herself. A Roman army sent to meet them suffered a disastrous defeat at the River Allia, a tributary of the Tiber. This left the city of Rome defenceless and open to attack. (The legendary Servian Wall, supposedly built by the Etruscan king Servius Tullus, was only partially built and would not be completed until after this attack by the Gauls and subsequent sack of the city.)
The citizens of Rome were evacuated to surrounding towns and the Gauls sacked and burned Rome. Only the strongly fortified and garrisoned settlement on the Capitoline Hill was able to resist the invaders. One of the many legends that make up traditional Roman historical literature tells of the sacred geese who, by their honking, alerted the defenders to an attempt by the Gauls to attack the hilltop fortress by stealth during the night. The Gauls stayed for seven months and left only after having been paid a large ransom in gold. Their departure may have been influenced by reports of an attack on their settlements further north by native Italian tribes.
During this time of crisis, The Etruscan city - state of Caere proved to be a vital friend to Rome. The people of Caere offered safekeeping for Rome's sacred objects and also brought military pressure upon the Gauls to leave the region.
The attack by the Gauls left Rome somewhat weakened and encouraged several previously subdued Italian tribes to rebel and seek their independence. These included the Etruscans, Volsci, Hernici, and Aequi. One by one, Rome defeated these tribes and reasserted her leadership over them during the next fifty years. In 338 B. C., Antium, chief city of the Volsci, was finally defeated.
Sinnigen and Boak pp. 47 - 48
Grant pp. 51 - 54
Atlas of the Roman World pp. 30 - 34