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
Conflicts on the Aisne - Submission of the Western Cantons
Caesar did not venture to give battle to the brave enemy six times as strong; to the north of the Aisne, not far from the modern Pontavert between Rheims and Laon, he pitched his camp on a plateau rendered almost unassailable on all sides partly by the river and by morasses, partly by fosses and redoubts, and contented himself with thwarting by defensive measures the attempts of the Belgae to cross the Aisne and thereby to cut him off from his communications. When he counted on the likelihood that the coalition would speedily collapse under its own weight, he had reckoned rightly.
King Galba was an honest man, held in universal respect; but he was not equal to the management of an army of 300,000 men on hostile soil. No progress was made, and provisions began to fail; discontent and dissension began to insinuate themselves into the camp of the confederates. The Bellovaci in particular, equal to the Suessiones in power, and already dissatisfied that the supreme command of the confederate army had not fallen to them, could no longer be detained after news had arrived that the Haedui as allies of the Romans were making preparations to enter the Bellovacic territory.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=75