The Boian War (238-236 BC) was a clash between Roman and a Gallic alliance led by the Boii that ended when the Gallic alliance fell apart.
The war came after a period of 45 years of peace between the Romans and the Gauls of northern Italy. The previous Gallic War of c.284-283 had seen the Romans defeat the Senones, whose homeland was on the Adriatic coast, and establish a new colony at Sena, near modern Ancona. This meant that Rome now had a border with the Boii, who lived on the northern Adriatic coast. It would appear to be the Boii who triggered this war - Polybius blamed it on the emergence of a new generation of Gauls who had no memory of the defeats suffered in the previous war, but the relevant book of Livy is lost so we don’t have any further details on what may have caused it. However in 268 the Romans had created a new colony at Ariminum (modern Rimini), further north along the coast from Sena, so some conflict between the new colonists and the nearby Boii seems likely.
The longest account of this war comes from Zonaras, a Byzantine historian writing over 1,100 years after the conflict, but using ancient sources for this period. Some details were recorded by Orosius, writing soon after 400 AD. Polybius’ account appears to miss the first two years of the war.
The Boii prepared for war by creating an alliance with a number of neighbouring tribes. The Roman response was led by Publius Valerius, one of the consuls for 238 BC. He fought two battles against the Gauls during the year, losing one and winning one. No location is given for either of these battles. According to Orosius 3,000 Romans were killed in the first battle. Reinforcements then arrived from Rome, and Valerius gained revenge for his defeat. Orosius records that 14,000 Gauls were killed and another 2,000 captured in this battle. Valerius returned to Rome where he was refused a Triumph because of the earlier defeat.
In the following year (237 BC) both consuls, Lucius Cornelius Lentulus Caudinus and Quintus Fulvius Flaccus campaigned against the Gauls. At first they stayed together, and were too strong for the Gauls, but they then separated in an attempt to pillage larger areas. This allowed the Gauls to concentrate against Flaccus, whose army was surrounded at night and attacked. The Romans were able to beat off this attack, but the Gauls then received reinforcements. The year appears to have ended in stalemate.
The third year of the war (236 BC) once again saw both consuls, Cornelius Lentulus Caudinus Publius and Caius Licinius Varus, campaign against the Gauls. However they were clearly badly outnumbered, and instead of risking a battle arranged an armistice and allowed Gallic envoys to go to Rome. These envoys appeared before the Senate, where they demanded the return the lands around Ariminum. The Senate refused these demands, and while the envoys were in Rome the Gallic army fell apart. The Boii’s allies turned on them and the alliance appears to have ended with a battle with heavy casualties on both Gallic sides. The survivors returned home, and the Boii were forced to agree to Roman terms, which included the surrender of a large part of their lands.
This war was fought at the same time as a war against the Ligurians in the north-west of Italy, although the two conflicts don’t appear to have been connected.
Another decade of peace followed, before war broke out again in 235, leading to the Roman victory at the battle of Telamon (225 BC).