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
Caesar and Ariovistus - Negotiations
Thus the threatening invasion of the Germans on the upper Rhine was obviated, and, at the same time, the party hostile to the Romans among the Celts was humbled. On the middle Rhine also, where the Germans had already crossed years ago, and where the power of Ariovistus which vied with that of Rome in Gaul was daily spreading, there was need of similar action, and the occasion for a rupture was easily found. In comparison with the yoke threatened or already imposed on them by Ariovistus, the Roman supremacy probably now appeared to the greater part of the Celts in this quarter the lesser evil; the minority, who retained their hatred of the Romans, had at least to keep silence.
A diet of the Celtic tribes of central Gaul, held under Roman influence, requested the Roman general in name of the Celtic nation for aid against the Germans. Caesar consented. At his suggestion the Haedui stopped the payment of the tribute stipulated to be paid to Ariovistus, and demanded back the hostages furnished; and when Ariovistus on account of this breach of treaty attacked the clients of Rome, Caesar took occasion thereby to enter into direct negotiation with him and specially to demand, in addition to the return of the hostages and a promise to keep peace with the Haedui, that Ariovistus should bind himself to allure no more Germans over the Rhine.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=68