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
Modifications of the Penal Law
To these measures having immediate reference to the great question of the proletariate there was added a series of enactments, which arose out of the general tendency to introduce principles milder and more accordant with the spirit of the age than the antiquated severity of the existing constitution. To this head belong the modifications in the military system. As to the length of the period of service there existed under the ancient law no other limit, except that no citizen was liable to ordinary service in the field before completing his seventeenth or after completing his forty-sixth year. When, in consequence of the occupation of Spain, the service began to become permanent,(13) it seems to have been first legally enacted that any one who had been in the field for six successive years acquired thereby a right to discharge, although this discharge did not protect him from being called out again afterwards.
13. Cf. III. VII. The Romans Maintain A Standing Army in Spain
At a later period, perhaps about the beginning of this century, the rule arose, that a service of twenty years in the infantry or ten years in the cavalry gave exemption from further military service.(14)
14. Thus the statement of Appian (Hisp. 78) that six years' service entitled a man to demand his discharge, may perhaps be reconciled with the better known statement of Polybius (vi. 19), respecting which Marquardt (Handbuch, vi. 381) has formed a correct judgment. The time, at which the two alterations were introduced, cannot be determined further, than that the first was probably in existence as early as 603 (Nitzsch, Gracchen, p. 231), and the second certainly as early as the time of Polybius. That Gracchus reduced the number of the legal years of service, seems to follow from Asconius in Cornel, p. 68; comp. Plutarch, Ti. Gracch. 16; Dio, Fr. 83, 7, Bekk.
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