The Imperial Roman Army was the military land force of the Roman Empire from 27 BC to 476 AD, and the final incarnation in the long history of the Roman army. This period is sometimes split into the Principate (27 BC – 284 AD) and the Dominate (284–476) periods. Under Augustus (r. 27 BC – AD 14), the army consisted of legions, eventually auxilia and also numeri. By the end of Augustus' reign, the imperial army numbered some 250,000 men, equally split between 25 legions and 250 units of auxiliaries. The numbers grew to a peak of about 450,000 by 211, in 33 legions and about 400 auxiliary units. By then, auxiliaries outnumbered legionaries substantially. From this peak, numbers probably underwent a steep decline by 270 due to plague and losses during multiple major barbarian invasions. Numbers were restored to their early 2nd-century level of c. 400,000 (but probably not to their 211 peak) under Diocletian (r. 284–305). After the Empire's borders became settled (on the Rhine-Danube line in Europe) by AD 68, virtually all military units (except the Praetorian Guard) were stationed on or near the borders, in roughly 17 of the 42 provinces of the empire in the reign of Hadrian (r. 117–138).
History
=== Background === The army of the late Republic that Augustus took over on becoming sole ruler of the Empire in 27 BC consisted of a number of large (5,000-strong) formations called legions, which were composed exclusively of heavy infantry. The legion's light infantry (velites) which had been deployed in earlier times (see Roman army of the mid-Republic), had been phased out as had its contingent of cavalry. Legions were recruited from Roman citizens only (i.e., from Italians and inhabitants of Roman colonies outside Italy), by regular conscription, although by 88 BC, a substantial proportion of recruits were volunteers. To remedy the deficiencies in capability of the legions (heavy and light cavalry, light infantry, archers and other specialists), the Romans relied on a motley array of irregular units of allied troops, both composed of subject natives of the empire's provinces (called the peregrini by the Romans) and of bands supplied, often on a mercenary basis, by Rome's allied kings beyond the Empire's borders. Led by their own aristocrats and equipped in their own traditional fashion, these native units varied widely in size, quality and reliability. Most would only be available for particular campaigns before returning home or disbanding.
Establishment
On gaining undisputed mastery over the Roman empire in 27 BC, Augustus (sole rule 27 BC – AD 14) was left with an army which was bloated by extraordinary recruitment for the Roman civil wars and at the same time lacking a suitable organisation for the defence and expansion of a vast empire. Even after disbanding most of his defeated adversary Mark Antony's legions, Augustus had 50 legions under his command, composed exclusively of Roman citizens i.e., by that time, of Italians and inhabitants of Roman colonies outside Italy. Alongside these were a mass of irregular non-Italian allied units whose command, size and equipment varied greatly. Some allied units came from provinces within the empire, others from beyond the imperial borders.
Legions
The first priority was to reduce the number of legions to a sustainable level. Fifty legions implied too high a recruitment burden for a male citizen-body only about two-million strong, especially as Augustus intended to create a long-term career force. The Emperor retained just over half his legions, disbanding the rest and settling their veterans in no less than 28 new Roman colonies. The number of legions remained close to that level throughout the Principate (varying between 25 and 33 in number). Unlike the Republican legions, which were, in theory at least, temporary citizen-levies for the duration of particular wars, Augustus and his right-hand man Agrippa clearly envisioned their legions as permanent units composed of career professionals. Under the late Republic, a Roman citizen iunior (i.e. male of military age: 16–46 years) could legally be required to serve a maximum of sixteen years in the legions and a maximum of six years consecutively. The average number of years served was about ten. In 13 BC, Augustus decreed sixteen years as the standard term of service for legionary recruits, with a further four years as reservists (evocati). In AD 5, the standard term was increased to twenty years plus five years in the reserves. In the period following its introduction, the new term was deeply unpopular with the troops. On Augustus' death in AD 14, the legions stationed on the rivers Rhine and Danube staged major mutinies, and demanded, among other things, reinstatement of a sixteen-year term. Augustus prohibited serving legionaries from marrying, a decree that remained in force for two centuries. This measure was probably prudent in the early imperial period, when most legionaries were from Italy or the Roman colonies on the Mediterranean, and were required to serve long years far from home. This could lead to disaffection if they left families behind. But from about AD 100 onwards, when most legions were based long-term in the same frontier-province and recruitment was primarily local, the prohibition of marriage became a legal encumbrance that was largely ignored. Many legionaries formed stable relationships and brought up families. Their sons, although illegitimate in Roman law and thus unable to inherit their fathers' citizenship, were nevertheless frequently admitted to legions. At the same time, the traditional grant of land to retiring veterans was made replaceable by a cash discharge bonus, as there was no longer sufficient state-owned land (ager publicus) in Italy to distribute. Unlike the Republic, which had relied primarily on conscription (i.e. compulsory levy), Augustus and Agrippa preferred volunteers for their professional legions. Given the onerous new term of service, it was necessary to offer a substantial bonus to attract sufficient citizen-recruits. In AD 5, the discharge bonus was set at 3,000 denarii. This was a generous sum equivalent to about 13 years' gross salary for a legionary of the time. To finance this major outlay, Augustus decreed a 5% tax on inheritances and 1% on auction-sales, to be paid into a dedicated aerarium militare (military treasury). However, veterans continued to be offered land instead of cash in Roman colonies established in the newly annexed frontier provinces, where public land was plentiful (as a result of confiscations from defeated indigenous tribes). This was another grievance behind the mutinies of 14 AD, as it effectively forced Italian veterans to settle far from their own country (or lose their bonus). The imperial authorities could not compromise on this issue, as the planting of colonies of Roman veterans was a crucial mechanism for controlling and Romanising a new province, and the foundation of veterans' colonies did not cease until the end of Trajan's rule (117). But as legionary recruitment became more localised (by AD 60, over half of recruits were not Italian-born), the issue became less relevant.
Augustus modified the command structure of the legion to reflect its new permanent, professional nature. In Republican tradition (but ever less in practice), each legion was under six equestrian military tribunes who took turns to command it in pairs. But in the late Republic, military tribunes were eclipsed by higher-ranking officers of senatorial rank called legati ("literally "envoys"). A proconsul (Republican governor) might ask the senate to appoint a number of legati to serve under him e.g., Julius Caesar, Augustus' grand-uncle and adoptive father, had five, and later ten, legati attached to his staff when he was governor of Cisalpine Gaul (58–51 BC). These commanded detachments of one or more legions at the governor's behest and played a critical role in the conquest of Gaul. But legions still lacked a single, permanent commander. This was provided by Augustus, who appointed a legatus to command each legion with a term of office of several years. The ranking senatorial military tribune (tribunus militum laticlavius) was designated deputy commander, while the remaining five equestrian tribunes served as the legatus' staff officers. In addition, Augustus established a new post of praefectus castrorum (literally "prefect of the camp"), to be filled by a Roman knight (often an outgoing centurio primus pilus, a legion's chief centurion, who was usually elevated to equestrian rank on completion of his single-year term of office). Technically, this officer ranked below the senatorial tribune, but his long operational experience made him the legion commander's de facto executive officer. The prefect's primary role was as the legion's quartermaster, in charge of legionary camps and supplies. It has been suggested that Augustus was responsible for establishing the small cavalry contingent of 120 horse attached to each legion. The existence of this unit is attested in Josephus' Bellum Iudaicum written after AD 70, and on a number of tombstones. The attribution to Augustus is based on the (unproven) assumption that legionary cavalry had completely disappeared in the Caesarian army. The Augustan era also saw the introduction of some items of more sophisticated and protective equipment for legionaries, primarily to improve their survival rate. The lorica segmentata (normally called simply "the lorica" by the Romans), was a special laminated-strip body-armour, was probably developed under Augustus. Its earliest depiction is on the Arch of Augustus at Susa (Western Alps), dating from 6 BC. The oval shield of the Republic was replaced by the convex rectangular shield (scutum) of the imperial era.
Auxilia
Augustus' ambitious expansion plans for the Empire (which included advancing the European border to the lines of the Elbe and Danube rivers) soon proved that 28 legions were not sufficient. Starting with the Cantabrian Wars, which aimed to annex the mineral-rich mountains of north-western Spain, Augustus' 41-year sole rule saw an almost uninterrupted series of major wars that frequently stretched the army's manpower to the limit. Augustus retained the services of numerous units of irregular allied native troops. But there was an urgent need for extra regular troops, organised, if not yet equipped, in the same way as the legions. These could only be drawn from the Empire's vast pool of non-citizen subjects, known as peregrini. These outnumbered Roman citizens by around nine to one in the early 1st century. The peregrini were now recruited into regular units of cohort-strength (c. 500 men), to form a non-citizen corps called the auxilia (literally: "supports"). By AD 23, Tacitus reports that the auxilia numbered roughly as many as the legionaries (i.e., c. 175,000 men). The roughly 250 regiments of auxilia this implies were divided into three types: an all-infantry cohors (plural: cohortes) (cohort) (c. 120 regiments); an infantry unit with a cavalry contingent attached, the cohors equitata (plural: cohortes equitatae) (80 units); and an all-cavalry ala (plural: alae, literal meaning: "wing"), of which c. 50 were originally established. It appears that at this early stage, auxiliary recruitment was ethnically based, with most men originating from the same tribe or province. Hence regiments carried an ethnic name e.g., cohors V Raetorum ("5th Cohort of Raeti"), recruited from the Raeti, a group of Alpine tribes that inhabited modern Switzerland. It has been suggested that the equipment of auxiliary regiments was not standardised until after AD 50, and that until then, auxiliaries were armed with the traditional weaponry of their tribe. But it is possible that at least some regiments had standardised equipment from Augustan times. Auxiliary regiments were designed to operate as a complement to the legions. That is, they performed exactly the same role as the Republic's alae of Italian allies (socii) before the Social War (91–88 BC), an equal number of which always accompanied legions on campaign.
Praetorian Guard
Under the late Republic, a proconsul on campaign often formed a small personal guard, selected from the troops under his command, known as a cohors praetoria ("commander's cohort"), from praetorium meaning the commander's tent at the centre of a Roman marching-camp (or commander's residence in a legionary fortress). At the Battle of Actium (31 BC), Augustus had five such cohorts around him. After the battle, he retained them in being as a permanent brigade in and around Rome, known as the praetoriani ("soldiers of the imperial palace"). Inscription evidence suggests that Augustus increased the Praetorian establishment to nine cohorts, each under the command of a tribunus militum (military tribune). With all the legions deployed in far-off provinces under the command of powerful senators, Augustus evidently considered that he needed a least one legion-sized force with him in Rome to deter potential usurpers. Augustus stationed three cohorts in the City itself, each housed in separate barracks, and the rest in neighbouring cities of Latium. Originally, each cohort was independent, but in 2 BC, Augustus appointed two overall commanders (praefecti praetorio) of equestrian rank, one for the cohorts based in the City, the other for those outside. Augustus envisaged the Praetorians as an elite force, whose duties included guarding the imperial palace on the Palatine Hill, protecting the Emperor's person and those of his family, defending the imperial government, and accompanying the emperor when he left the City on long journeys or to lead military campaigns in person. They also served as ceremonial troops on state occasions. Recruits to the ranks were, during the Julio-Claudian era, exclusively Italian-born. They were accorded much better pay and conditions than ordinary legionaries. In AD 5, the standard term of service for Praetorians was set at 16 years (compared to 25 years in the legions), and their pay was set at triple the rate of ordinary legionaries. In deference to Republican tradition, which banned armed men within the boundaries of the City of Rome, Augustus laid down a rule that Praetorians on duty within the City must not wear armour and must keep their weapons out of sight. Those Praetorians on important official duties, such as the Emperor's bodyguard-detail, wore the formal dress of Roman citizens, the toga, under which they concealed their swords and daggers. The rest wore the soldier's standard non-combat dress of tunic and cloak (paludamentum).
Urban cohorts
In addition to the Praetorians, Augustus established a second armed force in Rome, the cohortes urbanae ("urban cohorts"), of which three were based in the City and one in Lugdunum (Lyon) in Gaul, to protect the major imperial mint there. These battalions were tasked with maintaining public order in the City, including crowd-control at major events such as chariot-races and gladiatorial combats, and the suppression of the popular unrest that periodically shook the City, e.g., the riots caused by high grain prices in AD 19. Their command was given to the praefectus urbi, a senator who acted as Rome's "mayor". Unlike the praetorians, the urban cohorts were not deployed for military operations outside Italy.
Vigiles
The Vigiles or more properly the Vigiles Urbani ("watchmen of the City") or Cohortes Vigilum ("cohorts of the watchmen") were the firefighters and police of ancient Rome. The Vigiles also acted as a night watch, keeping an eye out for burglars and hunting down runaway slaves, and were on occasion used to maintain order in the streets. The Vigiles were considered a para-military unit and their organisation into cohorts and centuries reflects this.
Imperial German Bodyguard
To double-insure his own personal safety and that of imperial family members, Augustus established a small personal guard called the Germani corporis custodes (literally: "German bodyguards"). Probably of cohort-strength, these were crack horsemen recruited from native peoples on the lower Rhine, mainly from the Batavi. Their leader, probably a Batavi aristocrat, reported to the Emperor directly. The Germans shared the task of guarding the imperial family and the Palace with the Praetorians. In AD 68, the Emperor Galba disbanded the German Bodyguards because of their loyalty to Nero (r. 56–68 AD), whom he had overthrown. The decision caused deep offence to the Batavi, and contributed to the outbreak of the Revolt of the Batavi in the following year.
Principate
The dual-structure configuration of legions/auxilia established by Augustus remained essentially intact until the late 3rd century, with only minor modifications made during that long period. The senior officers of the army were, until the 3rd century, mainly from the Italian aristocracy. This was divided into two orders, the senatorial order (ordo senatorius), consisting of the c. 600 sitting members of the Roman Senate (plus their sons and grandsons), and the more numerous (several thousand-strong) equites equo publico or "knights granted a public horse", i.e., knights hereditary or appointed by the Emperor. Hereditary senators and knights combined military service with civilian posts, a career-path known as the cursus honorum, typically starting with a period of junior administrative posts in Rome, followed by five to ten years in the military and a final period of senior positions in either the provinces or at Rome. This tiny, tightly knit ruling oligarchy of under 10,000 men monopolised political, military and economic power in an empire of c. 60 million inhabitants and achieved a remarkable degree of political stability. During the first 200 years of its existence (30 BC – AD 180), the empire suffered only one major episode of civil strife (the Civil War of 68–9). Otherwise, attempts at usurpation by provincial governors were few and swiftly suppressed. Under the emperor Claudius (r. 41–54 AD), a minimum term of 25 years' service was established for auxiliary service (although many served for longer). On completion of the term, auxiliary soldiers, and their children, were from this time routinely granted Roman citizenship as a reward for service. (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 knightly rank, thus excluding serving centurions from such commands. The fact that auxiliary commanders were now all of the same social rank as all but one of a legion's military tribunes, probably indicates that auxilia now enjoyed greater prestige. Indigenous chiefs continued to command some auxiliary regiments, and were normally granted the rank of Roman knight for the purpose. It is also likely that auxiliary pay was standardised at this time, but pay scales during the Julio-Claudian period are uncertain. Estimates range from 33 to 50% of legionary pay, well below the 75-80% in force in the time of the emperor Domitian (ruled 81–96). Auxiliary uniform, armour, weapons and equipment were probably standardised by the end of the Julio-Claudian period (AD 68). Auxiliary equipment was broadly similar to that of the legions. By AD 68, there was little difference between most auxiliary infantry and their legionary counterparts in equipment, training and fighting capability. After about AD 80, the centuriae of the First Cohort of each legion were doubled in size to 160 men, but the number of centuriae apparently reduced to five, thus reducing the legion's centurions from 60 to 59. The legion's effectives were thus increased to c. 5,240 men plus officers. In the same period, some auxiliary regiments, both alae and cohortes, were also doubled to so-called milliaria size (literally "1,000-strong", actually only 720 in milliary alae and 800 in cohortes). But only a minority of auxiliary regiments, about one in seven, were so enlarged.
During the 2nd century some units with the new names numerus ("group") and vexillatio ("detachment") appear in the diploma record. Their size is uncertain, but was likely smaller than the regular alae and cohortes, as originally they were probably detachments from the latter, acquiring independent status after long-term separation. As these units are mentioned in diplomas, they were presumably part of the regular auxiliary organisation. But numeri was also a generic term used for barbarian units outside the regular auxilia. (see section 2.4 Irregular units, below). The traditional alternation between senior civilian and military posts fell into disuse in the late 2nd and 3rd centuries, as the Italian hereditary aristocracy was progressively replaced in the senior echelons of the army by the primipilares (former chief centurions). In the 3rd century, only 10% of auxiliary prefects whose origins are known were Italian equestrians, compared to the majority in the previous two centuries. At the same time, equestrians increasingly replaced the senatorial order in the top commands. Septimius Severus (r. 197–211 AD) placed equestrian primipilares in command of the three new legions he raised and Gallienus (r. 253–268 AD) did the same for all the other legions, giving them the title praefectus pro legato ("prefect acting as legate"). The rise of the primipilares may have provided the army with more professional leadership, but it increased military rebellions by ambitious generals. The 3rd century saw numerous coups d'état and civil wars. Few 3rd-century emperors enjoyed long reigns or died of natural causes.
Emperors responded to the increased insecurity with a steady build-up of the forces at their immediate disposal. These became known as the comitatus ("escort", from which derives the English word "committee"). To the Praetorian Guard's 10,000 men, Septimius Severus added the legion II Parthica. Based at Albano Laziale near Rome, it was the first legion to be stationed in Italy since Augustus. He doubled the size of the imperial escort cavalry, the equites singulares Augusti, to 2,000 by drawing select detachments from alae on the borders. His comitatus thus numbered some 17,000 men. The rule of Gallienus saw the appointment of a senior officer, with the title of dux equitum ("cavalry leader"), to command all the cavalry of the emperor's comitatus. This included equites promoti (cavalry contingents detached from the legions), plus Illyrian light cavalry (equites Dalmatarum) and allied barbarian cavalry (equites foederati). But the dux equitum did not command an independent "cavalry army", as was suggested by some more dated scholars. The cavalry remained integral to the mixed infantry- and cavalry-comitatus, with the infantry remaining the predominant element. The seminal development for the army in the early 3rd century was the Constitutio Antoniniana (Antonine Decree) of 212, issued by Emperor Caracalla (r. 211–217 AD). This granted Roman citizenship to all free inhabitants of the empire, ending the second-class status of the peregrini. This had the effect of breaking down the distinction between the citizen legions and the auxiliary regiments. In the 1st and 2nd centuries, the legions were the symbol (and guarantors) of the dominance of the Italian "master nation" over its subject peoples. In the 3rd century, they were no longer socially superior to their auxiliary counterparts (although they may have retained their elite status in military terms). In tandem, the legions' special armour and equipment (e.g., the lorica segmentata) was phased out during the early 3rd century. There was also a progressive reduction in the size of the legions. Legions were broken up into smaller units, as evidenced by the shrinkage and eventual abandonment of their traditional large bases, documented for example in Britain. In addition, from the 2nd century onwards, the separation of some detachments from their parent units became permanent in some cases, establishing new unit types, e.g., the vexillatio equitum Illyricorum based in Dacia in the early 2nd century and the equites promoti (legionary cavalry detached from their unit) and numerus Hnaufridi in Britain.
List of deployments
Notes: (1) Table excludes c. 4,000 officers (centurions and above). (2) Auxiliary cavalry nos. assumes 70% of cohortes were equitatae
The table shows the importance of auxiliary troops in the 2nd century, when they outnumbered legionaries by 1.5 to 1. The table shows that legions did not have a standard complement of auxiliary regiments and that there was no fixed ratio of auxiliary regiments to legions in each province. The ratio varied from six regiments per legion in Cappadocia to 40 per legion in Mauretania. Overall, cavalry represented about 20% (including the small contingents of legionary cavalry) of the total army effectives. But there were variations: in Mauretania the cavalry proportion was 28%. The figures show the massive deployments in Britannia and Dacia. Together, these two provinces account for 27% of the total auxilia corps.
Dominate
Units
Praetorian Guard
Augustus' successor Tiberius (r. 14–37 AD), appointed only single commanders for the Praetorian Guard: Sejanus (14–31) and, after ordering the latter's execution for treason, Macro. Under the influence of Sejanus, who also acted as his chief political advisor, Tiberius decided to concentrate the accommodation of all the Praetorian cohorts into a single, purpose-built fortress of massive size on the outskirts of Rome, beyond the Servian Wall. Known as the castra praetoria ("praetorian camp"), its construction was complete by AD 23. After Tiberius, the number of prefects in office simultaneously was normally two, but occasionally only one or even three. By AD 23, there were nine Praetorian cohorts in existence. These were probably the same size as legionary cohorts (480 men each), for a total of 4,320 effectives. Each cohort was under the command of a military tribune, normally a former chief centurion of a legion. It appears that each cohort contained some ninety cavalrymen who, like legionary cavalry were members of infantry centuriae, but operated in the field as three turmae of thirty men each. The number of Praetorian cohorts were increased to twelve by the time of Claudius. During the 68-9 civil war, Vitellius disbanded the existing cohorts because he did not trust their loyalty and recruited 16 new ones, all double-strength (i.e., containing 800 men each). However, Vespasian (r. 69–79 AD) reduced the number of cohorts back to the original nine (but still 800-strong), later increased to ten by his son, Domitian (r. 81–96 AD). By this time, therefore, the Guard consisted of c. 8,000 men. It was probably Trajan (r. 98–117 AD) who established a separate cavalry arm of the Guard, the equites singulares Augusti ("personal cavalry of the emperor", or imperial horseguards). An elite troop recruited from members of the finest auxiliary alae (originally from Batavi alae only), the singulares were tasked with escorting the emperor on campaign. The unit was organised as a milliary ala, probably containing 720 horsemen. It was under the command of a military tribune, who probably reported to one of the Praetorian prefects. It was the only praetorian regiment that admitted persons who were not natural-born citizens, although recruits appear to have been granted citizenship on enlistment and not on completion of 25 years' service as for other auxiliaries. The unit was housed in its own barracks on the Caelian Hill, separate from the main castra praetoria. By the time of Hadrian (r. 117–138 AD), the singulares appear to have numbered 1,000 men. They were further expanded to 2,000 horse in the early 3rd century by Septimius Severus, who constructed a new, larger base for them in Rome, the castra nova equitum singularium. By AD 100, therefore, the Guard consisted of c. 9,000 effectives, rising to c. 10,000 under Severus. Some historians have dismissed the Praetorian Guard as a parade-ground army of little military value. The Praetorians were certainly taunted as such by the soldiers of the Danubian legions during the civil war of 68–9. But Rankov argues that the Praetorians boasted a distinguished campaign-record that shows that their training and military effectiveness was far more impressive than those of merely ceremonial troops and amply justified their elite status. During the Julio-Claudian era (to 68), the Praetorians saw relatively little action in the field, as emperors only rarely led their armies in person. After that date, emperors led armies, and therefore deployed the Praetorians on campaign, much more frequently. The Praetorians were in the thick of the Emperor Domitian's wars, firstly in Germany and then on the Dacian front, where their prefect, Cornelius Fuscus was killed in action (87). Other examples include the Praetorians' prominent role in Trajan's Dacian Wars (101-6), as acknowledged on the friezes of Trajan's Column and the Adamklissi Tropaeum. Equally celebrated, on the Column of Marcus Aurelius, was the Praetorians' role in the Marcomannic Wars (166-80), in which two Guard prefects lost their lives. Even their final hour was wreathed in military glory: at the Battle of the Milvian Bridge (312), the Praetorians fought fiercely for their emperor Maxentius, trying to prevent the army of rival emperor Constantine I from crossing the river Tiber and entering Rome. Many perished fighting and others drowned when the makeshift pontoon-bridge they were using collapsed. Subsequently, the Praetorians paid the price of supporting the losing side: they were definitively disbanded, and their fortress demolished, by Constantine.
Legions
The legion consisted almost entirely of heavy infantry, i.e., infantry equipped with metal armour (helmets and cuirasses). Although it was almost unbeatable by non-Roman infantry on the battlefield, it was a large, inflexible unit that could not campaign independently due to the lack of cavalry cover and other specialist forces. It was dependent on the support of auxiliary regiments. The legion's basic sub-unit was the centuria (plural: centuriae), which literally means "a hundred men", but in practice numbered 80 men in the Principate, equivalent in numbers to half of a modern company. The legion's main tactical sub-unit was the cohors (plural: cohortes, or cohort), which contained six centuriae for a total of 480 men, roughly the same size as a modern battalion. There were 10 cohorts to each legion, or 4,800 men (c. 5,000 including the small legionary cavalry of 120 horse and officers). Thus, a legion was equivalent in numbers to a modern brigade. By AD 100, however, the legion's First Cohort was divided into only five centuriae, but double-strength at 160 men each, for a total of 800 men. At this point, therefore, a legion would have numbered c. 5,300 effectives. In addition, each legion contained a small cavalry contingent of 120 men. Unlike auxiliary cavalry, however, they do not appear to have been organised in separate cavalry squadrons (turmae) as were auxiliary cavalry, but to have been divided among specific centuriae. Legionary cavalry probably performed a non-combat role as messengers, scouts and escorts for senior officers.
=== Regular auxiliary units === The following table sets out the official, or establishment, strength of auxiliary units in the 2nd century. The real strength of a unit would fluctuate continually, but would likely have been somewhat less than the establishment most of the time.
Unless the regiment name was qualified by a specialist function e.g. cohors sagittariorum ("cohort of archers"), its infantry and cavalry were heavily equipped in the same way as the legionaries.
Cohors
These all-infantry units were modeled on the cohorts of the legions, with the same officers and sub-units. It is a common misconception that auxiliary cohortes contained light infantry: this only applies to specialist units such as archers. Their defensive equipment of regular auxiliary infantry was very similar to that of legionaries, consisting of metal helmet and metal cuirass (chain-mail or scale). There is no evidence that auxiliaries were equipped with the lorica segmentata, the elaborate and expensive laminated-strip body-armour that was issued to legionaries. However, legionaries often wore chain-mail and scalar cuirasses also. In addition, it appears that auxiliaries carried a round shield (clipeus) instead of the curved rectangular shield (scutum) of legionaries. As regards weapons, auxiliaries were equipped in the same way as legionaries: a javelin (although not the sophisticated pilum type provided to legionaries), a gladius (short stabbing-sword) and pugio (dagger). It has been estimated that the total weight of auxiliary infantry equipment was similar to that of legionaries', so that non-specialist cohortes may also be classified as heavy infantry, which fought in the battle-line alongside legionaries. There is no evidence that auxiliary infantry fought in a looser order than legionaries. It appears that in a set-piece battle-line, auxiliary infantry would normally be stationed on the flanks, with legionary infantry holding the centre, e.g., as in the Battle of Watling Street (AD 60), the final defeat of the rebel Britons under queen Boudicca. This was a tradition inherited from the Republic, when the precursors of auxiliary cohortes, the Latin alae, occupied the same position in the line. The flanks of the line required equal, if not greater, skill to hold as the centre.
==== Ala ==== The all-mounted alae contained the elite cavalry of the Roman army. They were specially trained in elaborate manoeuvres, such as those displayed to the emperor Hadrian during a documented inspection. They were best-suited for large-scale operations and battle, during which they acted as the primary cavalry escort for the legions, which had almost no cavalry of their own. They were heavily protected, with chain-mail or scale body armour, a cavalry version of the infantry helmet (with more protective features) and oval shield. Their offensive weapons included a spear (hasta), a cavalry sword (spatha), which was much longer than the infantry gladius to provide greater reach and a long dagger. The elite status of an alaris is shown by the fact that he received 20% greater pay than his counterpart in a cohort, and than a legionary infantryman.
==== Cohors equitata ==== These were cohortes with a cavalry contingent attached. There is evidence that their numbers expanded with the passage of time. Only about 40% of attested cohortes are specifically attested as equitatae in inscriptions, which is probably the original Augustan proportion. A study of units stationed in Syria in the mid 2nd century found that many units which did not carry the equitata title did in fact contain cavalrymen e.g., by discovery of a tombstone of a cavalryman attached to the cohort. This implies that by that time, at least 70% of cohortes were probably equitatae. The addition of cavalry to a cohort obviously enabled it to carry out a wider range of independent operations. A cohors equitata was in effect a self-contained mini-army. The traditional view of equites cohortales (the cavalry arm of cohortes equitatae), as expounded by G.L. Cheesman, was that they were just a mounted infantry with poor-quality horses. They would use their mounts simply to reach the battlefield and then would dismount to fight. This view is today discredited. Although it is clear that equites cohortales did not match equites alares (ala cavalrymen) in quality (hence their lower pay), the evidence is that they fought as cavalry in the same way as the alares and often alongside them. Their armour and weapons were the same as for the alares. Nevertheless, non-combat roles of the equites cohortales differed significantly from the alares. Non-combat roles such as despatch-riders (dispositi) were generally filled by cohort cavalry.
Specialised auxiliary units
Heavily armoured lancers
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. Their common feature was scalar armour which covered the whole body and conical helmets. Their lances (contus) were very long and were held in both hands, precluding the use of shields. In some cases, their horses are also depicted as protected by scalar armour, including head-piece. Normally, they were also equipped with long swords. In some cases, they carried bows instead of lances. 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 Ulpia contariorum and ala I Gallorum et Pannoniorum cataphractaria stationed in Pannonia and Moesia Inferior respectively in the 2nd century. Both faced the so-called "Sarmatian salient" between the Roman territories of Pannonia and Dacia, i.e., the Hungarian Plain, the territory of the Iazyges, a Sarmatian tribe which had migrated there and seized control of it during the 1st century.