So soon as Flaminius with his colleague had resigned the consulate, Marcellus was declared consul by the presiding officers called Interrexes; and, entering into the magistracy, chose Cnaeus Cornelius his colleague. There was a report that, the Gauls proposing a pacification, and the senate also inclining to peace, Marcellus inflamed the people to war; but a peace appears to have been agreed upon, which the Gaesatae broke; who, passing the Alps, stirred up the Insubrians (they being thirty thousand in number, and the Insubrians more numerous by far); and proud of their strength, marched directly to Acerrae, a city seated on the north of the river Po. From thence Britomartus, king of the Gaesatae, taking with him ten thousand soldiers, harassed the country round about. News of which being brought to Marcellus, leaving his colleague at Acerrae with the foot and all the heavy arms and a third part of the horse, and carrying with him the rest of the horse and six hundred light-armed foot, marching night and day without remission, he stayed not till he came up to these ten thousand near a Gaulish village called Clastidium, which not long before had been reduced under the Roman jurisdiction. Nor had he time to refresh his soldiers or to give them rest. For the barbarians, that were then present, immediately observed his approach, and contemned him, because he had very few foot with him. The Gauls were singularly skilful in horsemanship, and thought to excel in it; and as at present they also exceeded Marcellus in number, they made no account of him. They, therefore, with their king at their head, instantly charged upon him, as if they would trample him under their horses' feet, threatening all kinds of cruelties. Marcellus, because his men were few, that they might not be encompassed and charged on all sides by the enemy, extended his wings of horse, and, riding about, drew out his wings of foot in length, till he came near to the enemy. Just as he was in the act of turning round to face the enemy, it so happened that his horse, startled with their fierce look and their cries, gave back, and carried him forcibly aside. Fearing lest this accident, if converted into an omen, might discourage his soldiers, he quickly brought his horse round to confront the enemy, and made a gesture of adoration to the sun, as if he had wheeled about not by chance, but for a purpose of devotion. For it was customary to the Romans, when they offered worship to the gods, to turn round; and in this moment of meeting the enemy, he is said to have vowed the best of the arms to Jupiter Feretrius.
The king of the Gauls beholding Marcellus, and from the badges of his authority conjecturing him to be the general, advanced some way before his embattled army, and with a loud voice challenged him, and, brandishing his lance, fiercely ran in full career at him;exceeding the rest of the Gauls in stature, and with his armour, that was adorned with gold and silver and various colours, shining like lightning. These arms seeming to Marcellus, while he viewed the enemy's army drawn up in battalia, to be the best and fairest, and thinking them to be those he had vowed to Jupiter, he instantly ran upon the king, and pierced through his breastplate with his lance;then pressing upon him with the weight of his horse, threw him to the ground, and with two or three strokes more slew him. Immediately he leapt from his horse, laid his hand upon the dead king's arm and, looking up towards Heaven, thus spoke: "O Jupiter Feretrius, arbiter of the exploits of captains, and of the acts of commanders in war and battles, be thou witness that I, a general, have slain a general: I, a consul, have slain a king with my own hand, third of all the Romans; and that to thee I consecrate these first and most excellent of the spoils. Grant to us to despatch the relics of the war with the same course of fortune." Then the Roman horse joining battle not only with the enemy's horse, but also with the foot who attacked them, obtained a singular and unheard-of victory. For never before or since have so few horse defeated such numerous forces of horse and foot together. The enemies being to a great number slain, and the spoils collected, he returned to his colleague, who was conducting the war, with ill-success, against the enemies near the greatest and most populous of the Gallic cities, Milan. This was their capital, and, therefore, fighting valiantly in defence of it, they were not so much besieged by Cornelius, as they besieged him. But Marcellus having returned, and the Gaesatae retiring as soon as they were certified of the death of the king and the defeat of his army, Milan was taken. The rest of their towns, and all they had, the Gauls delivered up of their own accord to the Romans, and had peace upon equitable conditions granted to them.