ABSTRACT
The army of the principate had been a professional, long-service force. The men served for twenty-five years, or longer if they attained noncommissioned rank, and were for the most part recruited voluntarily. Service tended to be hereditary, as most of the soldiers followed in their fathers’ profession, and recruitment was very largely confined to the frontier areas where the bulk of the army was stationed. Apart from the praetorian guard, the army had consisted of thirty-odd legions of 6,000 men, infantry with a small cavalry element, and a much larger number of auxiliary units, infantry cohorts and cavalry alae, numbering generally 500, sometimes 1,000 men. The original distinction between the legions and the auxiliaries, that the former were Roman citizens, the latter provincials, ceased to exist in A.D. 212, when all free inhabitants of the empire became citizens, but the cohorts and alae continued to be inferior and less privileged units. The whole army was static, being distributed (except for the praetorians) in permanent camps or forts all round the frontiers. When a particular sector of the front needed reinforcement, detachments, normally 1,000 strong, were temporarily withdrawn from the legions elsewhere, which together with cohorts and alae withdrawn from the other provinces, formed a temporary expeditionary force.
