Rome defeats Etruscans and the Gauls (Boii) at the Battle of Lake Vadimo
After the end of the 3rd Samnite war, the Romans were forced to pacify the north once again. Although the Samnites were quiescent, for the moment, the Etruscans and Gauls were restive. Although the Etruscans were defeated previously, they had revolted and were threatening the city of Arretium, a Roman ally. The Romans, under praetor Lucius Caecilius, sent a consular army to relieve the city. They were crushed, losing their praetor along with 13,000 men.
Mommsen, Theodor. The History of Rome (Annotated) (Kindle Locations 8208-8210). Kindle Edition.
After the battle, the Romans sent an embassy to the Senones. This Gallic tribe was presumably an ally of Rome. But there were many Senones mercenaries among the Etruscan troops at Arretium. The Roman envoys demanded hostages. The Gallic chieftain had the envoys assassinated and declared war on Rome.
Along with the Senones, the Umbrians and the Etruscans were now at war with Rome.
The Romans, relentless as ever, sent another army north, under the command of the consul Publius Cornelius Dolabella. The Romans revenged themselves upon the Senones, killing many and driving the rest from Italy.
The Boii, threatened by the crushing of the Senones, declared war against Rome. The Romans were now faced with the Gauls (Boii), the Umbrians and the Etruscan League.
The combined alliance of three peoples formed a formidable army and marched on Rome. They were met at Lake Vadimo and decidedly defeated. The following year this alliance fought the Romans near Populonia with no better success. As a consequence, the Boii deserted their confederates and concluded a peace with the Romans. By this means the Romans were given a free hand in Lower Italy.
Mommsen, Theodor. The History of Rome (Annotated) (Kindle Locations 8210-8236). Kindle Edition.