The Auxilia (Latin: [au̯kˈs̠ɪlia], lit. "auxiliaries") were introduced as non-citizen troops attached to the citizen legions by Augustus after his reorganisation of the Imperial Roman army from 30 BC. By the 2nd century, the Auxilia contained the same number of infantry as the legions and, in addition, provided almost all of the Roman army's cavalry (especially light cavalry and archers) and more specialised troops. The auxilia thus represented three-fifths of Rome's regular land forces at that time. Like their legionary counterparts, auxiliary recruits were mostly volunteers, not conscripts.
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The Auxilia were mainly recruited from the peregrini, free provincial subjects who did not hold Roman citizenship and constituted the vast majority of the population in the 1st and 2nd centuries (c. 90% in the early 1st century). In contrast to the legions, which only admitted Roman citizens, members of the Auxilia could be recruited from territories outside of Roman control.
Reliance on the various contingents of non-Italic troops, especially cavalry, increased when the Roman Republic employed them in increasing numbers to support its legions after 200 BC. The Julio-Claudian period (30 BC–68 AD) saw the transformation of the Auxilia from motley levies to a standing corps with standardised structure, equipment and conditions of service. By the end of the period, there were no significant differences between legionaries and auxiliaries in terms of training, and thus, combat capability.
Auxiliary regiments were often stationed in provinces other than that in which they were originally raised, for reasons of security and to foster the process of Romanisation in 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.