The First Gallic Invasion of Italy of 390 B.C. was a pivotal event in the history of the Roman Republic and saw the city occupied and sacked for the last time in eight hundred years.
The Gauls had been established in the Po valley for some time by 390, but they had not yet appeared in Roman history. Livy mentions them towards the end of his account of the long siege of Veii (405-396 B.C.), giving the threat from the Gauls as the reason the other Etruscan cities failed to come to the aid of Veii.
The Gallic force that threatened Rome was the personal war band of Brennus, king of the Senones. Livy claims that the Gauls intended to settle in central Italy, but in fact Brennus and his men represented a raiding party rather than a migration. According to the Roman historical tradition Brennus was invited into Italy by Arruns of Clusium, who wanted to take revenge on his wife's lover. In the traditional version of this story the Gauls were still living on the far side of the Alps, but even Livy dismissed this idea. The rest of the story may well be true, at least in that Brennus was invited south by a faction in Clusium.
According to Livy the Romans send ambassadors to Clusium in an attempt to prevent war, but those ambassadors became involved in an argument with the Gauls and then took part in a battle against them, infuriating them. Abandoning their plans at Clusium the Gauls moved against Rome, catching the city totally unprepared to resist an attack. The Roman army of this period was a part-time amateur force, only raised when it was needed for a particular campaign. This time there was no time to raise the legions. A scratch force was put together, but it was brushed aside in the battle of the Allia (18 July 390 B.C.). Most of the defeated Roman troops fled to the recently conquered and well fortified city of Veii, leaving Rome undefended. The vestal virgins and the key religious icons of the city escaped to the friendly Etruscan city of Caere, close to the coast west of Rome, while those able bodied men left in the city prepared to defend the citadel on the Capitol.
The rest of the city was quickly occupied and sacked by the Gauls. It is possible that the Citadel also fell, but most Roman sources agree that this was not the case. Eventually the Gauls were paid off, taking 1000lbs of gold and leaving the city.
After leaving Rome the Gauls probably made their way to the south, where they were employed by Dionysius of Syracuse, who was involved in a war with the Greek cities of southern Italy (and with Caere - a few years later his fleet sacked that city's port at Pygri). On their way back to the north some of the Gauls were defeated by Caere at the Trausian Plain (location unknown), and some of the gold taken from Rome was recovered.
In the Roman historical tradition Furius Camillus, the hero of the siege of Veii, was in exile at Ardea when the Gauls attacked. Hearing that the Gauls were in Rome he raised an army, which reached the city before the ransom could be paid. The Gauls were driven out of Rome and annihilated in a battle somewhere outside the city. This part of the Roman tradition is often dismissed as complete fiction - it doesn't fit in at all with the Gaul's next appearance in the employment of Dionysius of Syracuse, and some Roman sources admit that the Gauls left with the gold. This does not automatically mean that Camillus was not involved in the process. If he was in exile, or absent from Rome for any reason, then there is no reason why he couldn't have raised an army outside the city. In that case the arrival or approach of Camillus's new army may have been the reason that the Gauls accepted a ransom, instead of continuing with the siege of the citadel.
In the traditional 'Varronian' chronology of Roman history the Gallic invasion and the sack of Rome are dated to 390 B.C. Evidence from Greek historians writing nearer in time to these events suggests that they actually took place in 387 or 386 B.C., but here we will follow the second edition of the Cambridge Ancient History in using the 'Varronian' date so that the events of the sack fall in the correct place in Roman history.