Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Measures Adopted to Check the Immigrations of the Transalpine Gauls
The chief efforts of the Romans in these regions were naturally directed to check the immigration of the Transalpine Celts, and to make the natural wall, which separates the peninsula from the interior of the continent, also its political boundary. That the terror of the Roman name had already penetrated to the adjacent Celtic cantons beyond the Alps, is shown not only by the totally passive attitude which they maintained during the annihilation or subjugation of their Cisalpine countrymen, but still more by the official disapproval and disavowal which the Transalpine cantons--we shall have to think primarily of the Helvetii (between the lake of Geneva and the Main) and the Carni or Taurisci (in Carinthia and Styria)--expressed to the envoys from Rome, who complained of the attempts made by isolated Celtic bands to settle peacefully on the Roman side of the Alps.
Not less significant was the humble spirit in which these same bands of emigrants first came to the Roman senate entreating an assignment of land, and then without remonstrance obeyed the rigorous order to return over the Alps (568-575), and allowed the town, which they had already founded not far from the later Aquileia, to be again destroyed. With wise severity the senate permitted no sort of exception to the principle that the gates of the Alps should be henceforth closed for the Celtic nation, and visited with heavy penalties those Roman subjects in Italy, who had instigated any such schemes of immigration.
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Reference address : https://www.ellopos.net/elpenor/rome/3-07-peace-hannibal-third-period.asp?pg=4