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
The Belgic League - The Maritime Cantons - The Leagues of Central Gaul
It has been already mentioned(20) that the Romans at the commencement of their Transalpine conquests found in the north a Britanno-Belgic league under the leadership of the Suessiones, and in central and southern Gaul the confederation of the Arverni, with which latter the Haedui, although having a weaker body of clients, carried on a rivalry.
20. Cf. IV. V. Transalpine Relations of Rome
In Caesar's time we find the Belgae in north-eastern Gaul between the Seine and the Rhine still forming such an association, which, however, apparently no longer extends to Britain; by their side there appears, in the modern Normandy and Brittany, the league of the Aremorican or the maritime cantons: in central or proper Gaul two parties as formerly contended for the hegemony, the one headed by the Haedui, the other by the Sequani after the Arvernians weakened by the wars with Rome had retired. These different confederacies subsisted independently side by side; the leading states of central Gaul appear never to have extended their clientship to the north-east nor, seriously, perhaps even to the north-west of Gaul.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=35