The siege of Acerrae (222 BC) was the first stage in a Roman attack on the Insubres of north-western Italy, and triggered a Gallic counter-attack on Clastidium which ended with a Gallic defeat, leaving the Insubres’ capital of Mediolanum vulnerable to attack (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.
Even so this defeat had clearly worried the Insubres, and at the start of 222 BC they sent ambassadors to Rome to sue for peace, but the consuls for the year, M. Claudius Marcellus and Gn. Cornelius Scipio Calvus, convinced the Senate not to accept. Instead the Consuls would be sent to invade the Insubres territory giving the first time consuls a chance to win military glory.
The Romans moved first, entering northern Italy and laying siege to Acerrae. This was probably at the site of modern Pizzighettone, on the Adda, so the Romans had been able to cross the Po without any problems. The town was well stocked with corn, but we don’t know much more about it. The Roman siege must have been a fairly major effort, as Polybius says that the Insubres were unable to come to the aid of the besieged town because the Romans had occupied all of the ‘advantageous positions’.
After the Romans rejected their request for peace, the Insubres had recruited about 30,000 Gaesatae, from the area around the Rhone. In an attempt to force the Romans to lift the siege of Acerrae, the Insubres now sent a force south of the Po to besiege Clastidium (Casteggio), about 34 miles to the west.
The Romans responding by splitting their army. Cornelius Scipio stayed at Acerrae with most of the infantry and two thirds of the cavalry while Marcellus was given the rest of the cavalry and 600 light infantry and sent to chase the Gauls. Marcellus defeated the Gauls somewhere between the Po and Clastidium, winning fame for killing a Gallic king in single combat.
Back at Acerrae Cornelius Scipio was able to capture Acerrae, which he then used as his base. In the aftermath of the fall of Acerrae the remaining Gauls retreated north-west to their main city of Mediolanum (Milan). Cornelius Scipio followed them, and was attacked outside the city. After some initial successes, the Romans were victorious, although the sources disagree on who deserved the credit for this victory. In its aftermath the Insubres surrendered and accepted Roman dominance, although this only lasted for a few years before Hannibal crossed the Alps and Roman power in the north of Italy crumbled.