Ab urbe condita
Titus Livius (Livy)
Livy. History of Rome, Volumes 1-2. Roberts, Canon, Rev, translator. London, New York: J. M. Dent and Sons; E. P. Dutton and Co., 1912.
He invited the surplus population of six tribes —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 canton of the Haedui, they accepted the omen of the place and built a city which they called Mediolanum.
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 tribe 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 tribe, I find, that came to Clusium, and from there to Rome;