Auxilia

From at least the fourth century BC Roman armies had been complemented by the troops of allied states. They supplied what Rome’s heavy infantry lacked: cavalry, archers, slingers, etcetera. Again from the early first century, some of those units became almost permanent professional additions to the armies of the rival warlords. For example, Caesar’s German cavalry, allies from the Gallic wars, came with him to Egypt during the Civil Wars almost a decade later. Rhodian and Balearic slingers or Cretan archers made a name for themselves as well. This policy continued in the Augustan era. Gradually the legionary core of the army was reinforced by professional auxiliary units, sometimes under direct Roman command, while others were still lead by tribal chiefs. Initially this process concerned mostly cavalry units and specialized light infantry. Later developments also saw formalized infantry units, modeled along the structure of the legionary cohort. Local militia and informally organized tribal units for single campaigns also had a role to play, which seems to have continued throughout the Principate, turning up as numeri in the epigraphical record. Further reinforcements were provided by client kingdoms. That these early units are often still called socii, again shows a reference to tradition in which allied states supplied troops according to their bilateral treaties with Rome. It is thus more due to the blurry Roman distinctions between ally, client kingdom and submitted tribe and a traditional outlook on the reaches of the actual empire itself. The numerical strength of these auxiliary units can only be estimated as, in Tacitus words, they were “according to the exigency of the moment, increased in number or were occasionally diminished.” Tacitus gives their number as “in not much inferior strength”, but his testimony is ambiguous. How these units were recruited varied. While non-citizen inhabitants from the Claudian era onward were enticed to enlist in auxiliary units with the promise of citizenship on retirement, this was not yet established practice during Augustus’ reign. Instead, auxiliary units were raised as complete units under command of tribal chiefs, who were responsible for keeping it up to strength. A somewhat irregular form of organization may have been common during Augustus’s reign, but more conventionally structured units of auxiliaries existed as well. This was surely the case for the auxiliary cohortes consisting of Roman citizens.