The battle of Clastidium (222 BC) saw the Romans defeat a Gallic army that had been sent to attack the city in an attempt to force the Romans to abandon their own siege of Acerrae (Telamon War). Instead the Gallic defeat left their capital of Mediolanum vulnerable to attack, and the Insubres were forced to surrender.
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 for the year, M. Claudius Marcellus and Gn. Cornelius Scipio Calvus, convinced the Senate to continue with the war. The two consuls then led their armies into Insubres territory, and laid siege to Acerrae (modern Pizzighettone, on the Adda).
After the reject of their peace offer, the Insubres had recruited 30,000 Gaesatae, Gallic mercenaries from the Rhone area of Transalpine Gaul. They now sent some of these men to besiege Clastidium (modern Casteggio), about 34 miles to the west of Acerrae, in the territory of Rome’s allies the Anares.
The Romans decided to split their army to try and deal with this new threat. Gn. Cornelius Scipio Calvus was left at Acerrae with most of the infantry and one third of the cavalry. Marcellus was given two-thirds of the cavalry and 600 light infantry and sent to chase the Gauls
There are several different accounts of the battle.
The earliest comes from Polybius. In his account the Gauls had begun the siege of Clastidium, but when they discovered that the Romans were coming lifted the siege and advanced to meet them. Polybius doesn’t say how large either force was.
As the Romans approached, the Gauls formed up in order of battle. Marcellus attacked with just his cavalry, and at first the Gauls held their own. However the Roman cavalry was then able to attack from their flanks and rear, and eventually the Gauls were defeated just by the cavalry. Many of the Gauls threw themselves into a nearby river (unnamed by Polybius), but more were killed by the Romans.
Plutarch produced a much longer account of the battle. His account gives us the size of Marcellus’s army, and also tells us that the Gallic force was made up of 10,000 Gaesatae. The Gauls weren’t worried by the Roman force, ignoring the small force of light infantry and outnumbering the Roman cavalry. The Gauls charged, with their king riding in the lead. In an attempt to make sure that his small force wasn't outflanked Marcellus extended his force out into a thin line and tried to outflank the Gauls. As this was happening his horse wheeled away from the oncoming Gauls. Marcellus was able to regain control of his horse, and pretended that he had carrying out a religious adoration of the sun. He then vowed to dedicate the most beautiful suit of armour amongst the Gauls to Iupiter Feretrius after the battle.
On the Gallic side the King, who was wearing a suit of armour decorated with gold, silver and embroideries, saw Marcellus and correctly judged him to be the Roman commander. He rode ahead of his men and challenged Marcellus to single combat. Marcellus charged, pierced the Gaul’s breastplate with his spear and the impact threw the Gaul to the ground, where Marcellus killed him. This made him the third Roman leader to claim the spolia opima, or rich spoils, stripping the arms and armour from an opposing commander he had killed in single combat (after Regulus and the fifth century general Aulus Cornelius Cossus). Marcellus prayed to Iupiter Feretrius and dedicated the spoils to the God. After that the main battle began, and his cavalry was able to defeat the Gallic cavalry and infantry.
Other sources give us the name of the Gallic king, Virdomarus or Vertomarus. This is the only mention of this king, but the leaders of the Gaesatae aren’t mentioned elsewhere.
Other sources generally agree that Marcellus killed the Gallic king and that the Romans were outnumbered.
Frontinus gives the most unusual account of the incident. In his version Marcellus blundered into the middle of the Gallic army and was desperately looking for a way to escape. He decided to attack, causing consternation and killing the Gallic leader.
Polybius doesn’t mention Marcellus again in his account of this war. However Plutarch has him rescuing Scipio, who was in trouble at Mediolanum, while Eutropius gives both consuls the credit.
Probably at about the same time as this battle was being fought, Scipio captured Acerrae, and then chose to attack the Insubres capital of Mediolanum. The exact details of the resulting battle are uncertain. Scipio advanced too far and the Gauls counterattacked, forcing him to retreat. In Polybius Scipio’s forces rallied and eventually forced the Gauls to retreat towards the mountains, leaving the city undefended. In Plutarch it was the arrival of Marcellus with the news of his victory at Clastidium that convinced the Insubres’s allies to retreat from the city, forcing them to surrender. In either case the Insubres had to submit to the Romans, although their control of northern Italy only lasted until Hannibal crossed the Alps.