The battle of Mediolanum (222 BC) saw a Gallic attempt to defeat a Roman army retiring from Mediolanum end in defeat, and was soon followed by the fall of Mediolanum and the surrender of the Insubres (Telamon War).
In 225 the Gauls had raised an army and attempted to attack Rome, but the Romans had had plenty of time to prepare, and were able to crush the Gauls between two consular armies at the battle of Telamon. In the aftermath of this victory, the Romans were able to campaign in the Po valley, crossing the river for the first time and forcing the Boii to submit. This only left the Insubres of north-western Italy, and in 223 the Romans were able to defeat them (probably near the River Clusius), before political infighting in Rome forced the Romans to withdraw.
At the start of 222 BC the Insubres sent ambassadors to Rome to sue for peace, but the consuls M. Claudius Marcellus and Gn. Cornelius Scipio Calvus were able to convince the Senate to continue the war. They then invaded Insubres territory, and laid siege to Acerrae (modern Pizzighettone, on the Adda).
The Gauls responded by sending a force to besiege Clastidium (modern Casteggio), about 34 miles to the west of Acerrae, in the territory of Rome’s allies the Anares. The Romans split their army, sending Marcellus with one thirds of the cavalry and a small force of infantry to lift that siege, while Cornelius Scipio was left at Acerrae with most of the infantry and the rest of the cavalry. Marcellus defeated the Gauls somewhere between Clastidium and the Po.
While Marcellus was away, Cornelius Scipio was able to capture Acerrae, capturing a large supply of corn. The surviving Gauls retreated north-west to Mediolanum (Milan), their main city. Cornelius Scipio gave chase, and was soon outside the city. However the Gauls refused to come out and fight, and the Romans had cleared advanced too quickly and were now exposed. Cornelius Scipio decided to return to Acerrae, but once he was on the move the Gauls sortied from Mediolanum and attacked.
Our sources give different accounts of the resulting battle.
Polybius, the earliest source, says that the Gauls made a bold attack on the Roman rear, killing a considerable number of Romans and forcing others to flee. Cornelius Scipio led his advanced guard back to the fight, rallying the fugitives as he went. The Gauls held on for a time, but eventually the Roman resistance wore them down and they fled the field. Instead of heading back to the city, the survivors fled to the mountains. In the aftermath of this victory, Cornelius Scipio was able to capture Mediolanum by assault. After this the chieftains of the Insubres surrendered.
In Plutarch Scipio attacked Mediolanum, but the Gauls put up such a vigorous defence that he was ‘less besieger than besieged’. However when Marcellus arrived with news of the death of the Gaesatae king at Clastidium the remaining Gaesatae withdrew from the city. Mediolanum was then taken by the Romans, and the Insubres surrendered the rest of their cities, gaining equitable peace terms.
The two accounts can fairly easily be combined. Plutarch’s reference to Scipio being ‘less besieger than besieged’ could be referring to the Gallic attack on Scipio’s army. Polybius doesn’t mention Marcellus, but also doesn’t say he didn’t join his colleague before the assault on the city.
In either case the fall of Mediolanum effectively ended the war. However the Gauls of northern Italy were by no means broken. In 218 they rebelled after the Romans planted colonies at Placentia and Cremona. The Romans had only just regained control (in fighting near Mutina) when the situation was entirely transformed by the arrival of Hannibal in northern Italy later in the year. After his early successes convinced the Gauls he posed a real threat to the Romans, many joined his army, and the Boii and Insubres were present at Hannibal’s greatest victory at Cannae.