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 Gallic Plan of War
But what were they to do? It was folly under such circumstances to let the matter come to the decision of arms; for these had already decidedly irrevocably. They might as well attempt to shake the Alps by throwing stones at them as to shake the legions by means of the Celtic bands, whether these might be congregated in huge masses or sacrificed in detail canton after canton. Vercingetorix despaired of defeating the Romans. He adopted a system of warfare similar to that by which Cassivellaunus had saved the insular Celts. The Roman infantry was not to be vanquished; but Caesar's cavalry consisted almost exclusively of the contingent of the Celtic nobility, and was practically dissolved by the general revolt.
It was possible for the insurrection, which was in fact essentially composed of the Celtic nobility, to develop such a superiority in this arm, that it could lay waste the land far and wide, burn down towns and villages, destroy the magazines, and endanger the supplies and the communications of the enemy, without his being able seriously to hinder it. Vercingetorix accordingly directed all his efforts to the increase of his cavalry, and of the infantry-archers who were according to the mode of fighting of that time regularly associated with it. He did not send the immense and self-obstructing masses of the militia of the line to their homes, but he did not allow them to face the enemy, and attempted to impart to them gradually some capacity of intrenching, marching, and manoeuvring, and some perception that the soldier is not destined merely for hand-to-hand combat.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=112