Titus Livius (c.59 BCE – AD 15), aka Livy, was a Roman historian from Padua (in modern northeast Italy) who drew upon the work of many previous authors. His information about the Celts seems to be derived from Polybius.
The following is an adaptation of Livy. History of Rome, trans. Rev. Canon Roberts. New York: E. P. Dutton and Co., 1912.
§ 5.33. Messengers came from Clusium begging for assistance against the Gauls. The tradition is that this nation, attracted by the report of the delicious fruits and especially of the wine — a novel pleasure to them — crossed the Alps and occupied the lands formerly cultivated by the Etruscans, and that Arruns of Clusium imported wine into Gaul in order to allure them into Italy. His wife had been seduced by a Lucumo, to whom he was guardian, and from whom, being a young man of considerable influence, it was impossible to get redress without getting help from abroad. In revenge, Arruns led the Gauls across the Alps and prompted them to attack Clusium.
I would not deny that the Gauls were conducted to Clusium by Arruns or some one else living there, but it is quite clear that those who attacked that city were not the first who crossed the Alps. As a matter of fact, Gauls crossed into Italy two centuries before they attacked Clusium and took Rome. Nor were the Clusines the first Etruscans with whom the Gaulish armies came into conflict; long before that they had fought many battles with the Etruscans who dwelt between the Apennines and the Alps. Before the Roman supremacy, the power of the Tuscans was widely extended both by sea and land. How far it extended over the two seas by which Italy is surrounded like an island is proved by the names, for the nations of Italy call the one the “Tuscan Sea,” from the general designation of the people, and the other the “Atriatic,” from Atria, a Tuscan colony. The Greeks also call them the “Tyrrhene” and the “Adriatic.” The districts stretching towards either sea were inhabited by them. They first settled on this side the Apennines by the western sea in twelve cities, afterwards they founded twelve colonies beyond the Apennines, corresponding to the number of the mother cities. These colonies held the whole of the country beyond the Po as far as the Alps, with the exception of the corner inhabited by the Veneti, who dwelt round an arm of the sea. The Alpine nations are undoubtedly of the same stock, especially the Raetii, who had through the nature of their country become so uncivilised that they retained no trace of their original condition except their language, and even this was not free from corruption.
§ 5.34. About the passage of the Gauls into Italy we have received the following account: while Tarquinius Priscus was king of Rome, the supreme power amongst the Celts, who formed a third part of the whole of Gaul, was in the hands of the Bituriges; they used to furnish the king for the whole Celtic race. Ambigatus was king at that time, a man eminent for his own personal courage and prosperity as much as for those of his dominions. During his sway the harvests were so abundant and the population increased so rapidly in Gaul that the government of such vast numbers seemed almost impossible. He was now an old man, and anxious to relieve his realm from the burden of over-population. With this view he signified his intention of sending his sister’s sons Bellovesus and Segovesus, both enterprising young men, to settle in whatever locality the gods should by augury assign to them. They were to invite as many as wished to accompany them, sufficient to prevent any nation from repelling their approach. When the auspices were taken, the Hercynian forest was assigned to Segovesus; to Bellovesus the gods gave the far pleasanter way into Italy. He invited the surplus population of six nations: the Bituriges, the Averni, the Senones, the Aedui, the Ambarri, the Carnutes, and the Aulerci. Starting with an enormous force of horse and foot, he came to the Tricastini.
Beyond stretched the barrier of the Alps, and I am not at all surprised that they appeared insurmountable, for they had never yet been surmounted by any route, as far at least as unbroken memory reaches, unless you choose to believe the fables about Hercules. Whilst the mountain heights kept the Gauls fenced in as it were there, and they were looking everywhere to see by what path they could cross the peaks which reached to heaven and so enter a new world, they were also prevented from advancing by a sense of religious obligation, for news came that some strangers in quest of territory were being attacked by the Salyi. These were Massilians who had sailed from Phocaea. The Gauls, looking upon this as an omen of their own fortunes, went to their assistance and enabled them to fortify the spot where they had first landed, without any interference from the Salyi. After crossing the Alps by the passes of the Taurini and the valley of the Douro, they defeated the Tuscans in battle not far from the Ticinus, and when they learnt that the country in which they had settled belonged to the Insubres, a name also borne by a district of the Aedui, they accepted the omen of the place and built a city which they called Mediolanum.
§ 5.35. Subsequently another body, consisting of the Cenomani, under the leadership of Elitovius, followed the track of the former and crossed the Alps by the same pass, with the goodwill of Bellovesus. They had their settlements where the cities of Brixia and Verona now stand. The Libui came next and the Saluvii; they settled near the ancient nation of the Ligurian Laevi, who lived about the Ticinus. Then the Boii and Lingones crossed the Pennine Alps, and as all the country between the Po and the Alps was occupied, they crossed the Po on rafts and expelled not only the Etruscans but the Umbrians as well. They remained, however, north of the Apennines. Then the Senones, the last to come, occupied the country from the Utis to the Aesis. It was this last nation, I find, that came to Clusium, and from there to Rome; but it is uncertain whether they came alone or helped by contingents from all the Cisalpine peoples. […]
§ 5.36. A peaceable enough mission, had it not contained envoys of a violent temper, more like Gauls than Romans. After they had delivered their instructions in the council of the Gauls, the following reply was given: “Although we are hearing the name of Romans for the first time, we believe nevertheless that you are brave men, since the Clusines are imploring your assistance in their time of danger. Since you prefer to protect your allies against us by negotiation rather than by armed force, we on our side do not reject the peace you offer, on condition that the Clusines cede to us Gauls, who are in need of land, a portion of that territory which they possess to a greater extent than they can cultivate. On any other conditions peace cannot be granted. We wish to receive their reply in your presence, and if territory is refused us we shall fight, while you are still here, that you may report to those at home how far the Gauls surpass all other men in courage.”
The Romans asked them what right they had to demand, under threat of war, territory from those who were its owners, and what business the Gauls had in Etruria. The haughty answer was returned that they carried their right in their weapons, and that everything belonged to the brave. Passions were kindled on both sides; they flew to arms and joined battle. Thereupon, contrary to the law of nations, the envoys seized their weapons, for the Fates were already urging Rome to its ruin. The fact of three of the noblest and bravest Romans fighting in the front line of the Etruscan army could not be concealed, so conspicuous was the valour of the strangers. And what was more, Quintis Fabius rode forward at a Gaulish chieftain, who was impetuously charging right at the Etruscan standards, ran his spear through his side and slew him. Whilst he was in the act of despoiling the body the Gauls recognised him, and the word was passed through the whole army that it was a Roman ambassador. Forgetting their rage against the Clusines, and breathing threats against the Romans, they sounded the retreat. […]