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
This unity of leadership acquired its full power through the efficiency of its instruments. Here the army comes, first of all, into view. It still numbered nine legions of infantry or at the most 50,000 men, all of whom however had faced the enemy and two-thirds had served in all the campaigns against the Celts. The cavalry consisted of German and Noric mercenaries, whose usefulness and trustworthiness had been proved in the war against Vercingetorix. The eight years' warfare, full of varied vicissitudes, against the Celtic nation--which was brave, although in a military point of view decidedly inferior to the Italian--had given Caesar the opportunity of organizing his army as he alone knew how to organize it.
The whole efficiency of the soldier presupposes physical vigour; in Caesar's levies more regard was had to the strength and activity of the recruits than to their means or their morals. But the serviceableness of an army, like that of any other machine, depends above all on the ease and quickness of its movements; the soldiers of Caesar attained a perfection rarely reached and probably never surpassed in their readiness for immediate departure at any time, and in the rapidity of their marching. Courage, of course, was valued above everything; Caesar practised with unrivalled mastery the art of stimulating martial emulation and the esprit de corps, so that the pre-eminence accorded to particular soldiers and divisions appeared even to those who were postponed as the necessary hierarchy of valour.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=3