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 Celtic Military System - Cavalry
The military capability of the nation felt the reflex influence of these political and social relations. The cavalry was throughout the predominant arm; alongside of which among the Belgae, and still more in the British islands, the old national war-chariots appear in remarkable perfection. These equally numerous and efficient bands of combatants on horseback and in chariots were formed from the nobility and its vassals; for the nobles had a genuine knightly delight in dogs and horses, and were at much expense to procure noble horses of foreign breed.
It is characteristic of the spirit and the mode of fighting of these nobles that, when the levy was called out, whoever could keep his seat on horseback, even the gray-haired old man, took the field, and that, when on the point of beginning a combat with an enemy of whom they made little account, they swore man by man that they would keep aloof from house and homestead, unless their band should charge at least twice through the enemy's line. Among the hired warriors the free-lance spirit prevailed with all its demoralized and stolid indifference towards their own life and that of others.
This is apparent from the stories-- however anecdotic their colouring--of the Celtic custom of tilting by way of sport and now and then fighting for life or death at a banquet, and of the usage (which prevailed among the Celts, and outdid even the Roman gladiatorial games) of selling themselves to be killed for a set sum of money or a number of casks of wine, and voluntarily accepting the fatal blow stretched on their shield before the eyes of the whole multitude.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=38