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
Subjugation of Gaul - Belgic Expedition
The foundations of the future edifice were laid; but in order to finish the building and completely to secure the recognition of the Roman rule by the Gauls, and that of the Rhine-frontier by the Germans, very much still remained to be done. All central Gaul indeed from the Roman frontier as far up as Chartres and Treves submitted without objection to the new ruler; and on the upper and middle Rhine also no attack was for the present to be apprehended from the Germans. But the northern provinces--as well the Aremorican cantons in Brittany and Normandy as the more powerful confederation of the Belgae--were not affected by the blows directed against central Gaul, and found no occasion to submit to the conqueror of Ariovistus.
Moreover, as was already remarked, very close relations subsisted between the Belgae and the Germans over the Rhine, and at the mouth of the Rhine also Germanic tribes made themselves ready to cross the stream. In consequence of this Caesar set out with his army, now increased to eight legions, in the spring of 697 against the Belgic cantons. Mindful of the brave and successful resistance which fifty years before they had with united strength presented to the Cimbri on the borders of their land,(38) and stimulated by the patriots who had fled to them in numbers from central Gaul, the confederacy of the Belgae sent their whole first levy--300,000 armed men under the leadership of Galba the king of the Suessiones--to their southern frontier to receive Caesar there.
38. Cf. IV. V. The Cimbri, Teutones, and Helvetii Unite
A single canton alone, that of the powerful Remi (about Rheims) discerned in this invasion of the foreigners an opportunity to shake off the rule which their neighbours the Suessiones exercised over them, and prepared to take up in the north the part which the Haedui had played in central Gaul. The Roman and the Belgic armies arrived in their territory almost at the same time.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=74