Titus Livius (Livy), The History of Rome, Book 5
Rev. Canon Roberts, Ed.

33. After¹ the expulsion of that citizen whose presence, if there is anything certain in human affairs, would have made the capture of Rome impossible, the doom of the fated City swiftly approached. Ambassadors came from Clusium begging for assistance against the Gauls. [2] 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. [3] 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. [4] 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. [5] As a matter of fact, Gauls crossed into Italy two centuries before they attacked Clusium and took Rome. [6] 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. [7] 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. [8] The Greeks also call them the ‘Tyrrhene’ and the ‘Adriatic.’ [9] 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. [10] 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. [11] The Alpine tribes 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.