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
This course was most natural for the Belgae, who were brought by neighbourhood and manifold intermixture into closer relation to the Germans who had crossed the Rhine, and moreover, with their less-developed culture, probably felt themselves at least as much akin to the Suebian of alien race as to their cultivated Allobrogian or Helvetic countryman.
But the southern Celts also, among whom now as already mentioned, the considerable canton of the Sequani (about Besangon) stood at the head of the party hostile to the Romans, had every reason at this very time to call in the Germans against the Romans who immediately threatened them; the remiss government of the senate and the signs of the revolution preparing in Rome, which had not remained unknown to the Celts, made this very moment seem suitable for ridding themselves of the Roman influence and primarily for humbling the Roman clients, the Haedui.
A rupture had taken place between the two cantons respecting the tolls on the Saone, which separated the territory of the Haedui from that of the Sequani, and about the year 683 the German prince Ariovistus with some 15,000 armed men had crossed the Rhine as condottiere of the Sequani.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=51