Auxilia

Non citizen troops in the Imperial Roman army

The auxilia (; ) were introduced as non citizen troops attached to the citizen legions by Augustus after his reorganisation of the Imperial Roman army from 27 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. 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 (27 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 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.

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). 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). 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. 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 ala, commanded by three Roman praefecti sociorum, 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 alae, 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). 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. 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 2 to 1. 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). 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 socii were all granted Roman citizenship, the Latin alae abolished, and the socii recruited into the legions. Furthermore, Roman equestrians were no longer required to perform cavalry service after this time. 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). 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. From 200 BC onwards, specialist troops were hired as mercenaries on a regular basis: sagittarii (archers) from Crete, and funditores (slingers) from the Balearic Isles almost always accompanied Roman legions in campaigns all over the Mediterranean. The other main sources of non Italian troops in the late Republic were subject provincials, allied cities and Rome's amici (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 socii militia legions.

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. 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 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. Cohortes were likely modelled on legionary cohorts i. e. six centuriae of about 80 men each (total about 480 men). Alae were divided into turmae (squadrons) of 30 (or 32) men, each under a decurio (literally: 'leader of ten'). This title derives from the old Roman cavalry of the pre Social War republic, in which each turma was under the command of three decuriones. Cohortes equitatae were infantry cohortes with a cavalry contingent of four turmae attached. Auxiliary regiments were now led by a 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 knightly rank, or a senior centurion. 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). 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. 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.

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. The German leader Arminius is the classic example at an individual level: after several years of serving in Rome's forces as prefect of an auxiliary unit, he used the military training and experience he had gained to lead a confederacy of German tribes against Rome, culminating in the destruction of three Roman legions in the Teutoberg Forest in 9 AD, and the abandonment of Augustus' strategy of annexing Germany as far as the Elbe river. (This strategy was never revived by later emperors). At a collective level, the risk was even greater, as the hugely dangerous Illyrian revolt proved. The central Illyrian tribes were tough and spartan shepherds of the Bosnian mountains and excellent soldier material. Their territory formed part of the strategic province of Illyricum, recently expanded to include the territory of the Pannonii, Celticised Illyrian tribes based on the west bank of the Danube who were subjugated by Rome in 12–9 BC (the Bellum Pannonicum). By the start of the Common Era, they were an important recruitment base for the auxilia. But discontent was festering among the Illyrian tribes, largely due to what they saw as the rapacity of Roman tax officials. In AD 6, several regiments of Dalmatae, a warlike Illyrian tribe, were ordered to report to a designated location to prepare to join Augustus' stepson and senior military commander Tiberius in a war against the Germans. Instead, they mutinied at the assembly point, and defeated a Roman force sent against them. The Dalmatae were soon joined by the Breuci, another Illyrian tribe that supplied several auxiliary regiments. They gave battle to a second Roman force from Moesia. They lost, but inflicted heavy casualties. The rebels were now joined by a large number of other Illyrian tribes. The Dalmatae attacked the port of Salona and overran the Adriatic coast, defeating a Roman force and exposing the Roman heartland of Italy to the fear of a rebel invasion. Augustus ordered Tiberius to break off operations in Germany and move his main army to Illyricum. When it became clear that even Tiberius' forces were insufficient, Augustus was obliged to raise a second task force under Tiberius' nephew Germanicus, resorting to the compulsory purchase and emancipation of thousands of slaves to find enough troops, for the first time since the aftermath of the Battle of Cannae over two centuries earlier. The Romans had now deployed no less than 15 legions and an equivalent number of auxilia. This amounts to a total of c. 150,000 men, including at least 50 auxiliary cohorts composed, exceptionally, of Roman citizens. These were men whose status or background was regarded by Augustus as unsuitable for recruitment into the legions: either natural born citizens of the lowest category, including vagrants and convicted criminals, or the freed slaves (Roman law accorded citizenship to the freed slaves of Roman citizens). These special units were accorded the title civium Romanorum ('of Roman citizens'), or c. R. for short. After the Illyrian revolt, these cohorts remained in being and recruited non citizens like other auxiliary units, but retained their prestigious c. R. title. In addition, the regular forces were assisted by a large number of allied troops from neighbouring Thrace deployed by their king Rhoemetalces I, a Roman amicus (puppet king). The Romans faced further reverses on the battlefield and a savage guerrilla war in the Bosnian mountains. It took them three years of hard fighting to quell the revolt, which was described by the Roman historian Suetonius, writing in c. AD 100, as the most difficult conflict faced by Rome since the Punic Wars over two centuries earlier. Despite the gravity of this rebellion, the Illyrians went on, alongside their neighbours the Thracians, to become the backbone of the Roman army. By the 2nd century, with roughly half the Roman army deployed on the Danube frontier, the auxilia and legions alike were dominated by Illyrian recruits. In the 3rd century, Illyrians largely replaced Italians in the senior officer echelons of praefecti of auxiliary regiments and tribuni militum of legions. Finally, from AD 268 to 379, virtually all emperors, including Diocletian and Constantine the Great, were Romanised Illyrians from the provinces of Dalmatia, Moesia Superior and Pannonia. These were members of a military aristocracy, outstanding soldiers who saved the empire from collapse in the turbulent late 3rd century.

Significant development of the Auxilia appears to have taken place during the rule of the emperor Claudius (41–54 AD). A minimum term of service of 25 years was established, at the end of which the retiring auxiliary soldier, and all his children, were awarded Roman citizenship. This is deduced from the fact that the first known Roman military diplomas date from the time of Claudius. This was a folding bronze tablet engraved with the details of the soldier's service record, which he could use to prove his citizenship. Claudius also decreed that prefects of auxiliary regiments must all be of equestrian rank, thus excluding centurions from such commands. Claudius annexed to the empire three regions that became important sources of auxiliary recruits: Britannia (43 AD), and the former client kingdoms of Mauretania (44) and Thracia (46). The latter became as important as Illyria as a source of auxiliary recruits, especially cavalry and archers. Britain in mid 2nd century contained the largest number of auxiliary regiments in any single province: about 60 out of about 400 (15%). They were a warlike people, skilled horsemen, boatmen and swimmers. In return for the unusual privilege of exemption from tributum (direct taxes on land and heads normally exacted from peregrini), they supplied a disproportionate number of recruits to the Julio Claudian auxilia: one ala and eight cohortes. They also provided most of Augustus' elite personal bodyguard unit (the Germani corpore custodes), which continued in service until 68 AD. The Batavi auxilia amounted to about 5,000 men, implying that during the entire Julio Claudian period, over 50% of all Batavi males reaching military age (16 years) may have enlisted in the auxilia. Thus the Batavi, although just 0.05% of the total population of the empire of c. 70 million in 23 AD, supplied about 4% of the total auxilia i. e. 80 times their proportionate share. They were regarded by the Romans as the very best (fortissimi, validissimi) of their auxiliary, and indeed all, their forces. In Roman service, both their cavalry and infantry had perfected a technique for swimming across rivers wearing full armour and weapons. Julius Civilis (, clearly a Latin name adopted on gaining Roman citizenship, not his native one) was a hereditary prince of the Batavi and the prefect of a Batavi cohort. A veteran of 25 years' service, he had distinguished himself by service in Britain, where he and the eight Batavi cohorts had played a crucial role in both the Roman invasion in 43 AD and the subsequent subjugation of southern Britain. By 69, however, Civilis, the Batavi regiments and the Batavi people had become utterly disaffected with Rome. After the Batavi regiments were withdrawn from Britain to Italy in 66, Civilis and his brother (also a prefect) were arrested by the governor of Germania Inferior on a fabricated accusation of sedition. The governor ordered his brother's execution, while Civilis, who as a Roman citizen had the right to appeal to the emperor, was sent to Rome in chains for judgement by Nero. He was released by Nero's overthrower and successor, Galba, but the latter also disbanded the imperial bodyguard unit for their loyalty to Nero. This alienated several hundred crack Batavi troops, and indeed the whole Batavi nation who regarded it as a grave insult. At the same time, relations collapsed between the Batavi cohorts and the legion to which they had been attached since the invasion of Britain 25 years earlier (XIV Gemina). Their mutual hatred erupted in open fighting on at least two occasions. At this juncture, the Roman empire was convulsed by its first major civil war since the Battle of Actium exactly a century earlier: the Year of the Four Emperors (69–70 AD). The governor of Germania Inferior, ordered to raise more troops, outraged the Batavi by attempting to conscript more Batavi than the maximum stipulated in their treaty. The brutality and corruption of the Roman recruiting centurions (including incidents of sexual assault on Batavi young men) brought already deep discontent in the Batavi homeland to the boil. Civilis now led his people in open revolt. Initially, he claimed he was supporting the bid for power of Vespasian, the general in command of the legions in Syria, whom Civilis had probably befriended when both were involved in the Roman invasion of Britain 25 years earlier (Vespasian was then commander of the legion II Augusta). But the uprising soon became a bid for independence. Civilis exploited the fact that some legions were absent from the Rhine area due to the civil war, and the rest under strength. In addition, the Roman commanders and their rank and file soldiers were divided by loyalty to rival emperors. Civilis quickly won the support of the Batavi's neighbours and kinsmen, the Cananefates, who in turn won over the Frisii. First the rebel allies captured two Roman forts in their territory, and a cohort of Tungri defected to Civilis. Then two legions sent against Civilis were defeated when their companion Batavi ala defected to his side. Most importantly, the eight Batavi cohorts stationed at Mainz with XIV Gemina mutinied and joined him, defeating at Bonn a Roman force that attempted to block their return to their homeland. By now, Civilis commanded at least 12 regiments (6,000 men) of Roman trained and equipped auxiliary troops, as well as a much larger number of tribal levies. A number of German tribes from beyond the Rhine joined his cause. Several other German and Gallic units sent against him deserted, as the revolt spread to the rest of Gallia Belgica, including the Tungri, Lingones and Treviri tribes. He was able to destroy the two remaining legions in Germania Inferior, (V Alaudae and XV Primigenia). By this stage, Rome's entire position on the Rhine and even in Gaul was imperiled. Their civil war over, the Romans mustered a huge task force of eight legions (five dispatched from Italy, two from Spain and one from Britain) to deal with Civilis. Its commander Petillius Cerialis had to fight two difficult battles, at Trier and Xanten, before he could overrun the Batavi's homeland. Tacitus' surviving narrative breaks off as he describes a meeting on an island in the Rhine delta between Civilis and Cerialis to discuss peace terms. We do not know the outcome of this meeting or Civilis' ultimate fate. But, in view of his former friendship with Vespasian, who had already offered him a pardon, and the fact that the Romans still needed the Batavi levies, it is likely that the terms were lenient by Roman standards. Petilius Cerialis took a number of reconstituted Batavi units with him to Britain, and the Batavi regiments continued to serve with special distinction in Britain and elsewhere for the rest of the 1st century and beyond. Even as late as 395, units with the Batavi name, although long since composed of recruits from all over the empire, were still classified as elite palatini, e. g. the equites Batavi seniores (cavalry) and auxilium Batavi seniores (infantry).

The revolt of the Batavi appears to have led to a significant change in the Roman government's policy on deployment of Auxilia. The revolt proved that in times of civil strife, when legions were far from their bases campaigning for rival claimants to the imperial throne, it was dangerous to leave provinces exclusively in the hands of auxiliary regiments recruited from the indigenous nation. During the Julio Claudian period, auxiliary regiments had often been deployed away from their original home province. The great majority of regiments probably founded in the 1st century were stationed away from their province of origin in the second e. g. of 13 British regiments recorded in the mid 2nd century, none were stationed in Britain. Furthermore, it appears that in the Flavian era native noblemen were no longer permitted to command auxiliary units from their own tribe. After a prolonged period in a foreign province a regiment would become assimilated, since the majority of its new recruits would be drawn from the province in which it was stationed, or neighbouring provinces. The Flavian period also saw the first formation of large, double size units, both infantry and cavalry, of a nominal strength of 1,000 men (cohors/ala milliaria), though they were actually mostly smaller (720 for an ala milliaria and 800 for a cohors milliaria).

In the 4th century, the Roman army underwent a radical restructuring. In the rule of Diocletian (284–305), the traditional Principate formations of legiones, alae and cohortes appear to have been broken up into smaller units, many of which bore a variety of new names. Under Constantine I (r. 312–337) it appears that military units were classified into three grades based on strategic role and to some extent quality: palatini, elite units normally part of the exercitus praesentales (imperial escort armies); comitatenses, higher grade interception forces based in frontier provinces; and limitanei, lower grade border troops. (See Late Roman army). The old Principate auxilia regiments provided the basis for units at all three grades. The Notitia Dignitatum lists about 70 alae and cohortes that retained their 2nd century names, mostly limitanei. But traces of other auxilia regiments can be found in the praesentales and comitatenses armies. For example, many of the new style auxilia palatina infantry regiments, considered among the best units in the army, were probably formed from old style auxiliary cohortes, which they appear to closely resemble. The late 4th century writer on military affairs Vegetius complains of contemporary young men joining the "auxilia" in preference to the "legions" to avoid the latter's tougher training and duties. But it is unclear what types of units he was referring to. It is possible that those older terms were still popularly used (misleadingly) to mean limitanei and comitatenses respectively. In any event, his quote in no way describes accurately the Principate auxilia, many of which were of very high quality. ** praefectus in Batavi and Tungri cohortes milliariae This would make an ala turma 32 men strong.</small>

In the Republican period, the standard trio of specialised auxilia were Balearic slingers, Cretan archers and Numidian light cavalry. These functions, plus some new ones, continued in the 2nd century auxilia.

Equites cataphractarii, or simply cataphractarii for short, were the heavily armoured cavalry of the Roman army. Based on Sarmatian and Parthian models, they were also known as contarii and clibanarii, although it is unclear whether these terms were interchangeable or whether they denoted variations in equipment or role. Together with new units of light mounted archers, the cataphractarii were designed to counter Parthian (and, in Pannonia, Sarmatian) battle tactics. Parthian armies consisted largely of cavalry. Their standard tactic was to use light mounted archers to weaken and break up the Roman infantry line, and then to rout it with a charge by the cataphractarii concentrated on the weakest point. The only special heavy cavalry units to appear in the 2nd century record are ala I Ulpia contariorum and ala I Gallorum et Pannoniorum cataphractaria stationed in Pannonia and Moesia Inferior, respectively, in the 2nd century.

From the Second Punic War until the 3rd century AD, the bulk of Rome's light cavalry (apart from mounted archers from Syria) was provided by the inhabitants of the provinces of Africa and Mauretania Caesariensis, the Numidae or Mauri (from whom derives the English term Moors), who were the ancestors of the Berber people of modern Algeria and Morocco. They were known as the equites Maurorum or Numidarum ('Moorish or Numidian cavalry'). On Trajan's Column, Mauri horsemen, depicted with long hair in dreadlocks, are shown riding their small but resilient horses bare back and unbridled, with a simple braided rope round their mount's neck for control. They wear no body or head armour, carrying only a small, round leather shield. Their weaponry cannot be discerned due to stone erosion, but is known from Livy to have consisted of several short javelins. Exceptionally fast and maneuverable, Numidian cavalry would harass the enemy by hit and run attacks, riding up and loosing volleys of javelins, then scattering faster than any opposing cavalry could pursue. They were superbly suited to scouting, harassment, ambush and pursuit. It is unclear what proportion of the Numidian cavalry were regular auxilia units as opposed to irregular foederati units. In the 3rd century, new formations of light cavalry appear, apparently recruited from the Danubian provinces: the equites Dalmatae ('Dalmatian cavalry'). Little is known about these, but they were prominent in the 4th century, with several units listed in the Notitia Dignitatum.

A unit of dromedarii ('camel mounted troops') is attested from the 2nd century, the ala I Ulpia dromedariorum milliaria in Syria.

A substantial number of auxiliary regiments (32, or about 1 in 12 in the 2nd century) were denoted sagittariorum, or archer units (from sagittarii , from sagitta 'arrow'). These 32 units (of which four were double strength) had a total official strength of 17,600 men. All three types of auxiliary regiment (ala, cohors and cohors equitata) could be denoted sagittariorum. Although these units evidently specialised in archery, it is uncertain from the available evidence whether all sagittariorum personnel were archers, or simply a higher proportion than in ordinary units. At the same time, ordinary regiments probably also possessed some archers, otherwise their capacity for independent operations would have been unduly constrained. Bas reliefs appear to show personnel in ordinary units employing bows. Three distinct types of archers are shown on Trajan's Column: (a) with scalar cuirass, conical steel helmet and cloak; (b) without armour, with cloth conical cap and long tunic; or (c) equipped in the same way as general auxiliary foot soldiers (apart from carrying bows instead of javelins). The first type were probably Syrian or Anatolian units; the third type probably Thracian. The standard bow used by Roman auxilia was the recurved composite bow, a sophisticated, compact and powerful weapon.

From about 218 BC onwards, the Republican army's slingers were exclusively mercenaries from the Balearic Islands, which had nurtured a strong indigenous tradition of slinging from prehistoric times. As a result, in classical Latin, Baleares (literally 'inhabitants of the Balearic Islands') became an alternative word for 'slingers' (funditores, from funda 'sling'). Because of this, it is uncertain whether most of the imperial army's slingers continued to be drawn from the Balearics themselves, or, like archers, derived mainly from other regions. Independent slinger units are not attested in the epigraphic record of the Principate. It is possible, however, that more than 20 such units served in Britain. The literal translation of numeri is 'numbers' and it was often used in the context of a generic title for any unit that was not of a standard size or structure. From the 2nd century onward they served as frontier guards, often supplied by the Sarmatians and the Germans. Little else is known about such units.

Throughout the Principate period, there is evidence of ethnic units of barbari outside the normal auxilia organisation fighting alongside Roman troops. To an extent, these units were simply a continuation of the old client king levies of the late Republic: ad hoc bodies of troops supplied by Rome's puppet petty kings on the imperial borders for particular campaigns. Some clearly remained in Roman service beyond the campaigns, keeping their own native leadership, attire and equipment and structure. These units were known to the Romans as socii ('allies'), symmachiarii (from symmachoi, Greek for 'allies') or foederati ('treaty troops' from foedus, 'treaty'). One estimate puts the number of foederati in the time of Trajan at about 11,000, divided into about 40 numeri (units) of about 300 men each. The purpose of employing foederati units was to use their specialist fighting skills. Many of these would have been troops of Numidian cavalry (see light cavalry above). The foederati make their first official appearance on Trajan's Column, where they are portrayed in a standardised manner, with long hair and beards, barefoot, stripped to the waist, wearing long trousers held up by wide belts and wielding clubs. In reality, several different tribes supported the Romans in the Dacian wars. Their attire and weapons would have varied widely. The Column stereotypes them with the appearance of a single tribe, probably the most outlandish looking, to differentiate them clearly from the regular auxilia. Judging by the frequency of their appearance in the Column's battle scenes, the foederati were important contributors to the Roman operations in Dacia. Another example of foederati are the 5,500 captured Sarmatian cavalrymen sent by Emperor Marcus Aurelius (r. 161–180) to garrison a fort on Hadrian's Wall after their defeat in the Marcomannic Wars.

Below centurion/decurion rank, junior officers in the Roman army were known as principales. An auxiliary cohort's ranks appear the same as in a legionary centuria. These were, in ascending order: tesserarius ('officer of the watch'), signifer (standard bearer for the centuria), optio (centurion's deputy) and vexillarius (standard bearer for the whole regiment, from vexillum). In the turmae of cohortes equitatae (and of alae? ), the decurion's second in command was probably known as a curator, responsible for horses and caparison. As in the legions, the principales, together with some regimental specialists, were classified in two pay scales: sesquiplicarii ('one and a half pay men') and duplicarii ('double pay men').

The limited evidence on auxiliary centuriones and decuriones is that such officers could be directly commissioned as well as promoted from the ranks. Many appear to have come from provincial aristocracies. Those rising from the ranks could be promotions from the legions as well as from the regiment's own ranks. In the Julio Claudian period auxiliary centuriones and decuriones were a roughly equal split between citizens and peregrini, though later citizens became predominant due to the spread of citizenship among military families. However, most of the surviving evidence concerns legionary centurions and it is uncertain whether their auxiliary counterparts shared their high status and non military role. From the time of emperor Claudius (r. 41–54) only Roman knights were eligible to hold command of an auxiliary regiment. This status could be obtained either by birth (i. e. if the person was the son of a hereditary Roman knight; or by attaining the property qualification (100,000 denarii, the equivalent of 400 years' gross salary for an auxiliary alaris); or by military promotion: the latter were the chief centurions of legions (centurio primus pilus) who would normally be elevated to equestrian rank by the emperor after completing their single year term as primuspilus. Equestrians by birth would normally begin their military careers at c. 30 years of age. An axillary had to do 25 years of service before joining the army. Commands were held in a set sequence, each held for 3–4 years: prefect of an auxiliary cohors, tribunus militum in a legion and finally prefect of an auxiliary ala. In Hadrian's time, a fourth command was added, for exceptionally able officers, of prefect of an ala milliaria. Like officers senatorial rank, hereditary equestrians held civilian posts before and after their decade of military service, whereas non hereditary officers tended to remain in the army, commanding various units in various provinces. By the 3rd century, most auxiliary prefects had exclusively military careers. The pay of a praefectus of an auxiliary regiment in the early 2nd century has been estimated at over 50 times that of a miles (common soldier). The reason for the huge gap between the top and the bottom of the pyramid is that Roman society was far more hierarchical than a modern one. A praefectus was not just a senior officer. He was also a Roman citizen (which most of his men were not) and, as a member of the equestrian order, an aristocrat. The social gulf between the praefectus and a peregrinus soldier was thus immense, and the pay differential reflected that fact.

The nomenclature of the great majority of regiments followed a standard configuration: unit type, followed by serial number, followed by name of the peregrini tribe (or nation) from whom the regiment was originally raised, in the genitive plural case e. g. cohors III Batavorum ('3rd Cohort of Batavi'); cohors I Brittonum ('1st Cohort of Britons'). Some regiments combine the names of two peregrini tribes, most likely after the merger of two previously separate regiments e. g. ala I Pannoniorum et Gallorum ('1st Wing of Pannonii and Gauls'). A minority of regiments are named after an individual, mostly after the first prefect of the regiment, e. g. ala Sulpicia (presumably named after a prefect whose middle (gens) name was Sulpicius). The latter is also an example of a regiment that did not have a serial number.

Regiments were often rewarded for meritorious service by the grant of an honorific title. The most sought after was the prestigious c. R. (civium Romanorum 'of Roman citizens') title. In the latter case, all the regiment's members at the time, but not their successors, would be granted Roman citizenship. But the regiment would retain the c. R. title in perpetuity. Another common title was the gens name of the emperor making the award (or founding the regiment) e. g. Ulpia: the gens name of Trajan (Marcus Ulpius Traianus r.98–117). Other titles were similar to those given to the legions e. g. pia fidelis (p. f. 'dutiful and loyal').