The (, ) were introduced as non-citizen troops attached to the citizen legions by Augustus after his reorganisation of the Imperial Roman army from 27 BC.Campbell, Duncan B. (2009). Roman Auxiliary Forts 27 BC-AD 378. Oxford: Osprey Publishing. By the 2nd century, the 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 thus represented three-fifths of Rome's regular land forces at that time. Like their legionary counterparts, auxiliary recruits were mostly volunteers, not conscripts.
The 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 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 (27 BC–68 AD) saw the transformation of the 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 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 Romanization 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.
Historical development
Background: Roman Republic (to 30 BC)
The mainstay of the Roman republic's war machine was the manipular legion, a heavy infantry unit suitable for close-quarter engagements on more or less any terrain, which was probably adopted sometime during the Samnite Wars (343–290 BC).Goldsworthy (2000) 44 Despite its formidable strength, the legion had a number of deficiencies, especially a lack of cavalry. Around 200 BC, a legion of 4,200 infantry had a cavalry arm of only 300 horse (just 7% of the total force).Goldsworthy (2000) 51 This was because the class of citizens who could afford to pay for their own horse and equipment – the equestrian order, the second rank in Roman society, after the senatorial order – was relatively small. In addition, the legion lacked missile forces such as slingers and archers.Goldsworthy (2000) 49 Until 200 BC, the bulk of a Roman army's cavalry was provided by Rome's regular Italian allies (socii), commonly known as the "Latin" allies, which made up the Roman military confederation. This was Rome's defence system until the Social War of 91–88 BC. The Italian forces were organised into alae (literally 'wings', because they were generally posted on the flanks of the Roman line of battle). An allied , commanded by three Roman , was similar or slightly larger in infantry size (4–5,000 men) to a legion, but contained a more substantial cavalry contingent: 900 horse, three times the legionary contingent. Since a pre-Social War consular army always contained an equal number of legions and , 75% of its cavalry was provided by the Latin allies. The overall cavalry element, c. 12% of the total force (2,400 out of a normal consular army of approximately 20,000 total effectives), was greater than in most peninsular Italian forces, but well below the overall 21% cavalry component that was typical of the Principate army (80,000 cavalry out of 380,000 total effectives in the early 2nd century).Holder (2003) 145Hassall (2000) 320
The Roman/Latin cavalry was sufficient while Rome was in conflict with other states in the mountainous Italian peninsula, which also disposed of limited cavalry resources. But, as Rome was confronted by external enemies that deployed far more powerful cavalry elements, such as the Gauls and the Carthaginians, the Roman deficiency in cavalry numbers could be a serious liability, which in the Second Punic War (218–202 BC) resulted in crushing defeats. Hannibal's major victories at the Trebia and at Cannae, were owed to his Spanish and Gallic heavy cavalry, which far outnumbered the Roman and Latin levies, and to his Numidians, light, fast cavalry which the Romans wholly lacked.Goldsworthy (2000) 74–5 The decisive Roman victory at Zama in 202 BC, which ended the war, owed much to the Numidian cavalry provided by king Massinissa, which outnumbered the Roman/Latin cavalry fielded by two to one.Goldsworthy (2000) 78–9 From then, Roman armies were always accompanied by large numbers of non-Italian cavalry: Numidian light cavalry and, later, Gallic heavy cavalry. For example, Caesar relied heavily on Gallic and German cavalry for his Conquest of Gaul (58–51 BC).Goldsworthy (2000) 126
As the role of native cavalry grew, that of Roman/Latin cavalry diminished. In the early 1st century BC, Roman cavalry was phased out altogether. After the Social War, the were all granted Roman citizenship, the Latin abolished, and the recruited into the legions.Goldsworthy (2000) 107 Furthermore, Roman equestrians were no longer required to perform cavalry service after this time.Keppie (1996) 372 The late Republican legion was thus probably bereft of cavalry (a tiny cavalry force of 120 men was probably added back to the legion under Augustus).Keppie (196) 375
By the outbreak of the Second Punic War, the Romans were remedying the legions' other deficiencies by using non-Italian specialised troops. Livy reports Hiero of Syracuse offering to supply Rome with archers and slingers in 217 BC.Livy Ab Urbe Condita XXII.37 From 200 BC onwards, specialist troops were hired as mercenaries on a regular basis: (archers) from Crete, and (slingers) from the Balearic Isles almost always accompanied Roman legions in campaigns all over the Mediterranean.G.L. Cheesman, The Auxilia of the Roman Imperial Army (Oxford, 1914), 8–9.
The other main sources of non-Italian troops in the late Republic were subject provincials, allied cities and Rome's (satellite kings). During the late Republic, non-Italian units were led by their own native chiefs, and their internal organisation was left to their own commanders. The units varied widely in dress, equipment, and weapons. They were normally raised for specific campaigns and often disbanded soon afterwards, in a similar manner to the earlier militia legions.Keppie (1996) 373
Foundation of the auxilia under Augustus (30 BC–14 AD)
It appears that not all indigenous units were disbanded at the end of the civil war period (31 BC). Some of the more experienced units were kept in existence to complement the legions, and became the core of the standing auxiliary forces that developed in the Julio-Claudian period.Keppie (1996) 379 During the early part of Augustus' rule (27 BC onwards), the corps of regular Auxilia was created. It was clearly inspired by the Latin forces of the pre-Social War Republic, as a corps of non-citizen troops parallel to the legions. But there were fundamental differences, the same as between Republican and Augustan legions. The Latin forces of the Republic were made up of part-time conscripts in units that would be raised and disbanded for and after particular campaigns. The Augustan Auxilia were mainly volunteer professionals serving in permanent units.
The unit structure of the Auxilia also differed from the Latin , which were like legions with a larger cavalry arm. However, Augustus organised the Auxilia into regiments the size of cohorts (a tenth the size of legions), due to the much greater flexibility of the smaller unit size. Further, the regiments were of three types: ala (cavalry), cohors (peditata) (infantry) and cohors equitata (mixed cavalry/infantry).Goldsworthy (2000) 127
The evidence for the size of the Augustus' new units is not clear-cut, with our most precise evidence dating to the 2nd century, by which time the unit strengths may have changed. were likely modelled on legionary cohorts i.e. six of about 80 men each (total about 480 men).Holder (1980) 7 were divided into (squadrons) of 30 (or 32) men, each under a (literally: 'leader of ten').Goldsworthy (2000) 214 This title derives from the old Roman cavalry of the pre-Social War republic, in which each was under the command of three .Goldsworthy (2003) 27 were infantry with a cavalry contingent of four attached.Holder (1980) 9
Auxiliary regiments were now led by a (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 knightly rank, or a senior centurion.Keppie (1996) 382
At the start of Augustus' sole rule (30 BC), the original core auxiliary units in the West were composed of warlike tribesmen from the Gallic provinces (especially Gallia Belgica, which then included the regions later separated to form the provinces Germania Inferior and Germania Superior), and from the Balkan provinces (Dalmatia and Illyricum). By 19 BC, the Cantabrian and Asturian Wars were concluded, leading to the annexation of northern Hispania and Lusitania. Judging by the names of attested auxiliary regiments, these parts of the Iberian peninsula soon became a major source of recruits.
Then the Danubian regions were annexed: Raetia (annexed 15 BC), Noricum (16 BC), Pannonia (9 BC) and Moesia (6 AD), becoming, with Illyricum, the Principate's most important source of auxiliary recruits for its entire duration. In the East, where the Syrians already provided the bulk of the Roman army's archers, Augustus annexed Galatia (25 BC) and Judaea: the former, a region in central Anatolia with a Celtic-speaking people, became an important source of recruits. In N. Africa, Egypt, Cyrene, and Numidia (25 BC) were added to the empire. Numidia (modern day Eastern Algeria) was home to the Numidians/Moors, the ancestors of today's Berber people. Their light cavalry (equites Maurorum) was highly prized and had alternately fought and assisted the Romans for well over two centuries: they now started to be recruited into the regular Auxilia. Even more Mauri units were formed after the annexation of Mauretania (NW Algeria, Morocco), the rest of the Berber homeland, in 44 AD by emperor Claudius (ruled 41–54).Holder (1982) 110–3
Recruitment was thus heavy throughout the Augustan period, with a steady increase in the number of units formed. By AD 23, the Roman historian Tacitus records that there were roughly the same numbers of auxiliaries in service as there were legionaries.Tacitus Annales IV.5 Since at this time there were 25 legions of c. 5,000 men each, the Auxilia thus amounted to c. 125,000 men, implying c. 250 auxiliary regiments.Goldsworthy (2003) 51
Illyrian revolt (6–9 AD)
During the early Julio-Claudian period, many auxiliary regiments raised in frontier provinces were stationed in or near their home provinces, except during periods of major crises such as the Cantabrian Wars, when they were deployed temporarily in theatre. This carried the obvious risk if their own tribe or ethnic group rebelled against Rome (or attacked the Roman frontier from outside the Empire), auxiliary troops could be tempted to make common cause with them. The Romans would then be faced by an enemy that included units fully equipped and trained by themselves, thus losing their usual tactical advantages over tribal foes.Keppie (1996) 396