Published online by Cambridge University Press: 09 August 2013
In 13 B.C. Augustus laid down that the minimum length of army service should be sixteen years. During the Late Republic it appears that at times only six years had been required of recruits, but it may be suspected that in the Civil Wars service had been lengthening—one can note already the gap of sixteen years between the great settlements of 30 and 14 B.C.
In A.D. 5, facing a shortage of recruits and an unwillingness of soldiers to remain in service beyond the statutory period, Augustus introduced a new package deal by which the praemia militiae were stepped up, perhaps considerably, but the minimum period of service extended to twenty years. Such are the figures given by Dio, but it is clear from the epigraphic and literary evidence—the latter provided in the main by Tacitus's account of the A.D. 14 mutinies—that Augustus sometimes succeeded in holding on to his soldiers beyond these limits, by retaining them as reservists sub vexillo, under a special standard, for a period of four years after 13 B.C., and for five years after A.D. 5, making totals of twenty and twenty-five years. Naturally this extension would not be stressed, and it should not be surprising if there is no mention in Dio of service sub vexillo.