The battle of the Trausian Plain (c.390-384 B.C.) probably saw an Etruscan army from the city of Caere defeat all or part of the Gallic war band that was responsible for the sack of Rome (traditionally dated to 390 B.C.).
The exact sequence of events after the sack of Rome is unclear. In the Roman tradition the Gauls were defeated by a new Roman army led by Furius Camillus soon after leaving the city, but this was probably a later Roman invention.
The historian Justin, writing at least six hundred years later but summarising an earlier work reported that they moved south and signed on as mercenaries serving Dionysius of Syracuse, who was then involved in a war against the Greeks of southern Italy, as well as against Caere.
Both Strabo and Diodorus Siculus report a battle in which an army from Caere defeated part of the Gallic war band. According to Strabo the battle took place in the country of the Sabini, soon after the Gauls left Rome. The Caeretans recovered the gold taken from Rome and returned it to the city. They had already sheltered the Vestal Virgins (mentioned in Livy), their sacred treasures and some of the population of Rome, and they were rewarded with honorary citizenship.
Diodorus Siculus gives the location of the battle as the Trausian Plain (an unknown location), and reports that it took place when some of the Gauls were returning from the south. This would fit in with Justin's account - the Gauls could have been returning south after leaving the service of Dionysius or he could have sent them north to attack Caere. Like Livy and the Roman tradition Diodorus also records a victory won by Furius Camillus over part of the Gallic army, this time while it was involved in a siege.
Although these accounts differ in some details a rough outline of events can be reconstructed, in which Caere, one of Rome's allies, managed to defeat at least part of the Gallic army at some time after the sack of the city. This victory was probably later used as the basis of the Roman tradition's account of a victory that saw the Gauls destroyed by Camillus, although it may also be a distorted account of the defeat of the Second Gallic Invasion of Italy of 367 B.C., also credited to Camillus.