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
By the side of these mounted warriors the infantry fell into the background. In the main it essentially resembled the bands of Celts, with whom the Romans had fought in Italy and Spain. The large shield was, as then, the principal weapon of defence; among the offensive arms, on the other hand, the long thrusting lance now played the chief part in room of the sword. Where several cantons waged war in league, they naturally encamped and fought clan against clan; there is no trace of their giving to the levy of each canton military organization and forming smaller and more regular tactical subdivisions.
A long train of waggons still dragged the baggage of the Celtic army; instead of an entrenched camp, such as the Romans pitched every night, the poor substitute of a barricade of waggons still sufficed. In the case of certain cantons, such as the Nervii, the efficiency of their infantry is noticed as exceptional; it is remarkable that these had no cavalry, and perhaps were not even a Celtic but an immigrant German tribe. But in general the Celtic infantry of this period appears as an unwarlike and unwieldy levy en masse; most of all in the more southern provinces, where along with barbarism valour had also disappeared. The Celt, says Caesar, ventures not to face the German in battle. The Roman general passed a censure still more severe than this judgment on the Celtic infantry, seeing that, after having become acquainted with them in his first campaign, he never again employed them in connection with Roman infantry.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=39