All this, however, required time. For once the Romans had allowed themselves to be surprised, and it was too late at least to save Etruria. The Celts found the Apennines hardly defended, and plundered unopposed the rich plains of the Tuscan territory, which for long had seen no enemy. They were already at Clusium, three days' march from Rome, when the army of Ariminum, under the consul Papus, appeared on their flank, while the Etruscan militia, which after crossing the Apennines had assembled in rear of the Gauls, followed the line of the enemy's march. Suddenly one evening, after the two armies had already encamped and the bivouac fires were kindled, the Celtic infantry again broke up and retreated on the road towards Faesulae (Fiesole): the cavalry occupied the advanced posts during the night, and followed the main force next morning. When the Tuscan militia, who had pitched their camp close upon the enemy, became aware of his departure, they imagined that the host had begun to disperse, and marched hastily in pursuit. The Gauls had reckoned on this very result: their infantry, which had rested and was drawn up in order, awaited on a well-chosen battlefield the Roman militia, which came up from its forced march fatigued and disordered. Six thousand men fell after a furious combat, and the rest of the militia, which had been compelled to seek refuge on a hill, would have perished, had not the consular army appeared just in time. This induced the Gauls to return homeward. Their dexterously-contrived plan for preventing the union of the two Roman armies and annihilating the weaker in detail, had only been partially successful; now it seemed to them advisable first of all to place in security their considerable booty. For the sake of an easier line of march they proceeded from the district of Chiusi, where they were, to the level coast, and were marching along the shore, when they found an unexpected obstacle in the way. It was the Sardinian legions, which had landed at Pisae; and, when they arrived too late to obstruct the passage of the Apennines, had immediately put themselves in motion and were advancing along the coast in a direction opposite to the march of the Gauls. Near Telamon (at the mouth of the Ombrone) they met with the enemy. While the Roman infantry advanced with close front along the great road, the cavalry, led by the consul Gaius Atilius Regulus in person, made a side movement so as to take the Gauls in flank, and to acquaint the other Roman army under Papus as soon as possible with their arrival. A hot cavalry engagement took place, in which along with many brave Romans Regulus fell; but he had not sacrificed his life in vain: his object was gained. Papus became aware of the conflict, and guessed how matters stood; he hastily arrayed his legions, and on both sides the Celtic host was now pressed by Roman legions. Courageously it made its dispositions for the double conflict, the Transalpine Gauls and Insubres against the troops of Papus, the Alpine Taurisci and the Boii against the Sardinian infantry; the cavalry combat pursued its course apart on the flank. The forces were in numbers not unequally matched, and the desperate position of the Gauls impelled them to the most obstinate resistance. But the Transalpine Gauls, accustomed only to close fighting, gave way before the missiles of the Roman skirmishers; in the hand-to-hand combat the better temper of the Roman weapons placed the Gauls at a disadvantage; and at last an attack in flank by the victorious Roman cavalry decided the day. The Celtic horsemen made their escape; the infantry, wedged in between the sea and the three Roman armies, had no means of flight. 10,000 Celts, with their king Concolitanus, were taken prisoners; 40,000 others lay dead on the field of battle; Aneroestus and his attendants had, after the Celtic fashion, put themselves to death.

The victory was complete, and the Romans were firmly resolved to prevent the recurrence of such surprises by the complete subjugation of the Celts on the south of the Alps. In the following year (530) the Boii submitted without resistance along with the Lingones; and in the year after that (531) the Anares; so that the plain as far as the Po was in the hands of the Romans. The conquest of the northern bank of the river cost a more serious struggle. Gaius Flaminius crossed the river in the newly-acquired territory of the Anares (somewhere near Piacenza) in 531; but during the crossing, and still more while making good his footing on the other bank, he suffered so heavy losses and found himself with the river in his rear in so dangerous a position, that he made a capitulation with the enemy to secure a free retreat, which the Insubres foolishly conceded. Scarce, however, had he escaped when he appeared in the territory of the Cenomani, and, united with them, advanced for the second time from the north into the canton of the Insubres. The Gauls perceived what was now the object of the Romans, when it was too late: they took from the temple of their goddess the golden standards called the "immovable", and with their whole levy, 50,000 strong, they offered battle to the Romans. The situation of the latter was critical: they were stationed with their back to a river (perhaps the Oglio), separated from home by the enemy's territory, and left to depend for aid in battle as well as for their line of retreat on the uncertain friendship of the Cenomani. There was, however, no choice. The Gauls fighting in the Roman ranks were placed on the left bank of the stream; on the right, opposite to the Insubres, the legions were drawn up, and the bridges were broken down that they might not be assailed, at least in the rear, by their dubious allies.

In this way undoubtedly the river cut off their retreat, and their way homeward lay through the hostile army. But the superiority of the Roman arms and of Roman discipline achieved the victory, and the army cut its way through: once more the Roman tactics had redeemed the blunders of the general. The victory was due to the soldiers and officers, not to the generals, who gained a triumph only through popular favour in opposition to the just decree of the senate. Gladly would the Insubres have made peace; but Rome required unconditional subjection, and things had not yet come to that pass. They tried to maintain their ground with the help of their northern kinsmen; and, with 30,000 mercenaries whom they had raised amongst these and their own levy, they received the two consular armies advancing once more in the following year (532) from the territory of the Cenomani to invade their land. Various obstinate combats took place; in a diversion, attempted by the Insubres against the Roman fortress of Clastidium (Casteggio, below Pavia), on the right bank of the Po, the Gallic king Virdumarus fell by the hand of the consul Marcus Marcellus. But, after a battle already half won by the Celts but ultimately decided in favour of the Romans, the consul Gnaeus Scipio took by assault Mediolanum, the capital of the Insubres, and the capture of that town and of Comum terminated their resistance. Thus the Celts of Italy were completely vanquished, and as, just before, the Romans had shown to the Hellenes in the war with the pirates the difference between a Roman and a Greek sovereignty of the seas, so they had now brilliantly demonstrated that Rome knew how to defend the gates of Italy against freebooters on land otherwise than Macedonia had guarded the gates of Greece, and that in spite of all internal quarrels Italy presented as united a front to the national foe, as Greece exhibited distraction and discord.