Auxiliary troops were mainly recruited from the peregrini, i.e. free provincial subjects of the Roman Empire who did not hold Roman citizenship and constituted the vast majority of the empire's population in the 1st and 2nd centuries (ca. 90% in the early 1st century). Auxiliaries also included some Roman citizens and probably barbarians (barbari, as the Romans called peoples located outside the Empire's borders). This was in contrast to the legions, which only admitted Roman citizens.
The auxilia developed from the varied contingents of non-Italian troops, especially cavalry, that the Roman Republic used in increasing numbers to support its legions after 200d BC. The Julio-Claudian period (30r BC–68b AD) saw the transformation of these motley temporary levies into a standing corps of regiments with standardised structure, equipment and conditions of service. By the end of this period, there were no significant differences between legionaries and most auxiliaries in terms of training, or thus combat capability.
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Auxiliary regiments were often stationed in provinces other than the province in which they were originally raised, both for reasons of imperial security and to foster the process of Romanisation and integration of the provinces. The regimental names of many auxiliary units persisted into the 4th century, but by then the units in question were different in size, structure, and quality from their predecessors.
Auxiliary regiments were now led by one praefectus (prefect), who could be either a native nobleman, who would probably be granted Roman citizenship for the purpose (e.g. the famous German war leader Arminius gained Roman citizenship probably by serving as an auxiliary prefect before turning against Rome); or a Roman, either of equestrian rank, or a senior centurion.[21]
