The battle of Arretium (283 BC) was a Roman defeat at the hands of a Gallic army that was besieging the city of Arretium in Tuscany, and that triggered a Roman response that destroyed the power of the Senones (Gallic War of 284-283 BC).
Arretium (Arezzo in modern Tuscany) was originally an Etruscan town, but it had been captured by the Romans in 311 BC. Polybius says that it was attacked by the Gauls, but not by which tribe. However the Senones, who had led the Gallic army that defeated the Romans at the battle of the Allia (c.387) and gone on to sack Rome held an area just to the east on the Adriatic coast, so are the most likely candidates.
According to Polybius the Romans sent an army under the Praetor Lucius Caecilius to lift the siege. This may be the same man as the consul for 284 BC Lucius Caecilius Metellus Denter – although it was rare for a consul to serve as a praetor in the following year it wasn’t unknown.
Polybius simply states that the Romans attacked the Gauls outside the town, were defeated, and Lucius Caecilius was killed. The much later Christian historian Orosius expanded this into a major defeat with seven military tribunes and 30,000 soldiers being killed, but his account is unlikely to be accurate as that would have made it one of the largest defeats in Roman history!
In the aftermath of this defeat Manius Curius was placed in command and sent legates to negotiate the return of the prisoners. These legates were murdered, triggering a Roman invasion of the Senones homeland, in which the Senones were defeated in battle (Battle of Lake Vadimon), their homeland was devastated and settled by the Romans, who established their first colony in a former Celtic area at Sena.
The Periochae of Livy reverses the order of events, and has the envoys killed first, followed by the death of Lucius Caecilius.
Although this is a poorly documented battle, the war it was part of was significant in two ways. First, it was the last time that the Senones inflicted a defeat of the Romans (and the war was the last time they are recorded fighting against them). Second, it saw the Romans establish their first colony on former Gallic lands, the start of a process that would see them slowly conquer all of Cisalpine Gaul.