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
Political Significance of the Marian Military Reform
This complete revolution in the constitution of the Roman army seems certainly in substance to have originated from purely military motives; and on the whole to have been not so much the work of an individual, least of all of a man of calculating ambition, as the remodelling which the force of circumstances enjoined in arrangements which had become untenable. It is probable that the introduction of the system of inland enlistment by Marius saved the state in a military point of view from destruction, just as several centuries afterwards Arbogast and Stilicho prolonged its existence for a time by the introduction of foreign enlistment. Nevertheless, it involved a complete--although not yet developed--political revolution.
The republican constitution was essentially based on the view that the citizen was at the same time a soldier, and that the soldier was above all a citizen; there was an end of it, so soon as a soldier-class was formed. To this issue the new system of drill, with its routine borrowed from the professional gladiator, could not but lead; the military service became gradually a profession. Far more rapid was the effect of the admission--though but limited--of the proletariate to participate in military service; especially in connection with the primitive maxims, which conceded to the general an arbitrary right of rewarding his soldiers compatible only with very solid republican institutions, and gave to the capable and successful soldier a sort of title to demand from the general a share of the moveable spoil and from the stale a portion of the soil that had been won.
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=13