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 system of a reserve was already foreshadowed in the earlier employment of the cavalry, but it was now completely developed by the partition of the army into three divisions and the reservation of the flower of the veterans for the last and decisive shock.
While the Greek phalanx had developed the close, and the Oriental squadrons of horse armed with bows and light missile spears the distant, modes of fighting respectively, the Roman combination of the heavy javelin with the sword produced results similar, as has justly been remarked, to those attained in modern warfare by the introduction of bayonet-muskets; the volley of javelins prepared the way for the sword encounter, exactly in the same way as a volley of musketry now precedes a charge with the bayonet.
Lastly, the elaborate system of encampment allowed the Romans to combine the advantages of defensive and offensive war and to decline or give battle according to circumstances, and in the latter case to fight under the ramparts of their camp just as under the walls of a fortress--the Roman, says a Roman proverb, conquers by sitting still.
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Reference address : https://www.ellopos.net/elpenor/rome/2-08-law-religion-army-economy-nationality.asp?pg=22