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The power of the Roman emperors rested on their control of massive armed forces, paid for out of the emperor's privy purse and bound to him by an oath of personal allegiance. The vast majority of soldiers were stationed in so called imperial provinces, commanded by direct appointees of the emperor. The Roman imperial army was thus in effect very much a private army.
The imperial army was a standing professional army. It contained both conscripts and volunteers serving a minimum term of sixteen years, though most had to serve for 25 years or more before they were up for retirement. To preserve the loyalty of the soldiers on which their position of power rested, the emperors looked well after their interests. Pay was regular and comparatively generous and on occasion supplemented by donativa, special bonuses of up to five years pay. On completion of their term of service soldiers received a large retirement grant of thirteen to seventeen years' worth of pay. In addition to these monetary rewards serving soldiers and retired veterans were also granted numerous legal privileges.
The army of the empire was like its republican predecessors made up of a variety of different units. The most important divisions were however made between the legions, the auxiliaries, the fleet and the imperial guards. It are these main branches of the armed services that will be briefly discussed below.
The core of the Roman army was formed by the units called legions from the latin legio, meaning a levy. During the first three centuries of the empire the army contained no more than 25 to 34 legions. Each of these units consisted of about 5000-6000 men recruited among the citizen body. Although the soldiers of the legion were Roman citizens, this did not imply that they originated from the city of Rome or even Italy. With the spread of the franchise among the population of the conquered territories provincials quickly became the most important source of recruits. Italian levies however remained the most usual source for newly raised legions, although several units were formed using marines or legionaries detached from existing formations.
The legio was a miniature army that contained within its ranks troops trained and equipped to perform all kinds of different duties both on and off the battlefield. Although the vast majority of soldiers served as heavy infantry, other legionaries fought as cavalry, archers or light infantry. Other specialised troops operated artillery consisting of torsion guns. The troops were however not solely prepared for combat. Legionaries regularly served as combat engineers constructing fortifications, roads and bridges. As the legion counted among its complement a vast number of men with special skills it was in many ways selfsupporting. A large part of its military equipment could be produced by artisans in the ranks. Soldiers trained as surveyors, engineers and architects ensured that the legion needed little outside help for its building requirements. Administrative duties were performed by other legionaries both within their unit as well as in the provincial bureaucracy.
Each legion carried a number and a name, e.g. legio X Gemina (the tenth 'twin' or 'double' legion), to which honorary titles like pia fidelis (dutiful and loyal) could be added. The numbering and naming of units followed no rationalised pattern. As many of the formations originated in the various armies of the civil wars following the death of Iulius Caesar, several legions carried identical numerals or nicknames. Even new legions that were levied were named and numbered according to diverse systems. The sense of individuality provided by these numbers and titles was reinforced by the use of different unit symbols and signs like bulls, boars or capricorns.
The strength and organisation of the legions varied in time and was probably not completely standardised throughout the army. Generally speaking however the legio was organised in ten cohortes or cohorts. These cohorts consisted each of three manipuli, literally 'handfuls', which were in their turn subdivided in two centuriae or 'hundreds'. These centuriae were composed of a number of contubernia or 'tentparties'. Although the name centuria would seem to indicate a unit of a hundred soldiers, this unit could comprise anything from 30 to over 200 individuals. The usual establishment strength however is thought to have been 80 men. From the second half of the first century AD in at least some of the legions the first cohort was reorganised in five double strength centuriae while the remainder continued to be organised in the old manner.
In addition to the regular organisation of cohortes, manipuli and centuriae of the legionary heavy infantry there were other subunits for the equites legionis, the legionary cavalry, and the antesignani or lancearii, the elite legionary light infantry. The exact details of their organisation are as yet not very clear. For a variety of duties provisional units known as vexillationes or numeri were formed. The strength and organisation of these provisional units varied greatly and was only in part based on the more regular subdivisions of the legion.
Command of the legion was usually given to a legatus legionis picked by the emperor from the senatorial class who generally had some previous military experience through service as a tribunus. In Egypt and from the the start of the third century also in other provinces the command was not entrusted to a senatorial legatus, but to a praefectus legionis, an acting commander drawn from the equestrian order. The legionary commander was assisted by six military tribunes. With the exception of the units stationed in Egypt one of these tribuni was usually a young senator at the start of his public career. Known as a tribunus laticlavius from the broad purple stripes on his tunic this senior tribune was second-in-command. His collegues from the equestrian order were known as tribuni angusticlavii and generally had done earlier service as a commander of an auxiliary infantry unit. A former senior centurion usually performed the duties of praefectus castrorum, camp commandant, and was the third in the chain of command.
The most important officers in the legions were the centuriones. These men were partly directly recruited from the Roman knights or the city councilmembers, but the greater part of the centurions had previously served as soldiers and NCO's in the legions or the praetorian cohorts. Depending on the organisation of the legion either sixty or fifty nine centuriones ordinarii commanded the centuriae, while a varying number of centuriones supernumerarii were employed for special duties. These officers were known by titles derived from the place of their units in the old battle order. The hastatus prior, princeps prior and pilus prior were the higher ranking officers commanding the manipuli. The hastatus posterior, princeps posterior and pilus posterior acted as their deputies. The cohorts were under the command of the pilus prior. Distinguished from their fellow officers were the primi ordines, the senior centurions of the first cohort of the legion. These men had achieved their posts by prior service in other postings and were chief advisors of the legionary commanders. The post of primus pilus, the highest ranking centurion in the legion, carried great prestige and assured entry into the equestrian order.
An uncertain number of supernumerary centurions performed a variety of tasks both within the legion itself and in other units. Centuriones exercitatores for example were used as training officers for the legionary cavalry and the horse guards of provincial governors and the emperor. A centurio stratorum was employed to oversee the remount system of the provincial armies and on occasion to command the singulares, the auxiliary soldiers serving as a governor's guard. The centuriones lanceariorum led the elite legionary infantry known as antesignani or lancearii. Other supernumerary officers performed duties in the medical service of the legions.
To assist the officers the legion counted a number of NCO's among its strength. These men were known as principales and depending on their status recieved as duplicarii double pay or as sesquiplicarii pay and a half. Each centurio ordinarius had an optio as his deputy. Whereas the centurion led his men from the front, the optio was stationed at the rear of the unit to keep the legionaries from shirking away in combat. The signifer or standard bearer carried the signum of the unit. This standard served both as a rallying point for the soldiers and to communicate simple visual commands to the troops in battle. The task of carrying the signum in battle was dangerous as the soldier had to stand in the first rank and could carry only a small buckler. It may not be strictly coincidental that available epigraphical evidence contains a relatively large number of discentes signiferorum, trainee standardbearers. The signifer also assumed responsibility for the financial administration of the unit and functioned as the legionaries' banker. The tesserarius was a third NCO attached to a centuria and in charge of the distribution and collection of the watch words. Both optio and signifer received double pay, but the tesserarius attached to a centuria was on pay and a half. Other principales like the cornicularius were attached to the administrative offices of the legion.
A considerable number of legionary soldiers were classed as immunes. These men were exempted from the more tedious chores because of the special tasks they had to perform, but received no extra pay. As many a soldier without immunity was forced to bribe his centurion to escape the less desirable duties, the immunes would in practice have had some financial gain from their position. Among the immunes were musicians, military police, cavalry troopers, drill and weapons instructors, artisans, clerks and medical orderlies. It was usual for both immunes and principales to have served several years as a munifex, a private liable for all kinds of duty and fatigues, before they received promotion. Most, if not all, positions were reached after a period of specialised training as a discens.
The imperial Roman army continued the republican tradition of supplementing the citizen legions with units recruited from peregrini, non-citizens from conquered or allied communities. In the imperial army the total numerical strength of the various auxiliary formations was roughly comparable to that of the legionary troops. These forces were known as socii or auxilia and were composed of both regular and irregular formations. Many modern works distinguish regular auxilia consisting of cohortes and alae from irregular numeri. This present day division disregards the fact that irregular units could be designated as a cohors and that numerus was a very generic term which was also in general use for regular army units. As in the legiones draftees and volunteers served side by side in the auxiliary forces. With the spread of Roman citizenship among the population of the conquered territories the auxilia were increasingly recruiting citizens into the ranks, blurring the original division between peregrine auxiliaries and citizen legionaries.
The imperial auxilia were composed of a variety of units. Infantry units were generally organised in cohorts that in the case of cohortes equitatae could include a small mounted force. Cavalry was usually formed into alae or 'wings'. Both cohortes and alae could comprise either quingenaria units of approximately 500 man or milliaria formations of 800-1000 soldiers. Infantry cohorts with a mounted contingent had an additional 120 to 250 cavalry troopers. Infantry cohorts were composed of three to five manipuli of each two centuriae. Cavalry alae counted 16 to 24 turmae of 30-40 mounted soldiers. Auxiliary formations were usually commanded by a praefectus cohortis or praefectus alae, though a tribunus cohortis or legionary centurio was occasionally employed. Some of these commanders were drawn from the tribal aristocracy, though most were recruited from the equestrian order. Command of a cavalry alae was only entrusted to men who had previously served as a praefectus cohortis and legionary tribunus. The infantry subunits had similar officers and NCO's as their legionary counterparts. Cavalry turmae were placed under a decurio instead of a centurion. Legionaries were regularly transferred to act as officers and NCO's in the units of the auxilia.
Units in the auxiliary forces carried like the legions a number and a title. The numbering of units followed different patterns and partly reflected the order in which troops had been levied. The names of units varied greatly, many like cohors I Batavorum being derived from the tribe that provided the original levies, others reflecting the armament, e.g. the ala I contariorum, or honouring a former commander, for example ala Siliana. Redeployment of units and the Roman practice of local recruitment of replacements meant that the ethnic titles borne by formations did not reflect the actual origins of its soldiers.
The infantry of the auxilia consisted mainly of soldiers trained and equipped to fight in a way comparable to that of the legionary heavy infantry. In addition to these existed specialised formations of light infantry adept at fighting in a looser order. Units of archers formed a large proportion of the available auxiliary forces. The alae were for the larger part made up of medium cavalry suited for both skirmishing and shock tactics. Formations of mounted archers were also much employed. A minority of the cavalry units were composed of heavy cavalry troopers armed with the contus, a two handed cavalry spear. These soldiers and some of their mounts as well were heavily armoured. In at least part of the medium cavalry alae a number of troopers used to fight as horse archers or heavy cavalry giving the unit a wider range of combat capabilities.
From the auxiliary units of a provincial army a number of soldiers were selected for service in the singulares of the governor's guard. Infantrymen from the cohorts were grouped in the pedites singulares while horsemen from both cohortes equitatae and alae were brigaded in the equites singulares. Both units were trained and commanded by legionary centuriones. The strength of these guard formations was probably related to the numbers of troops deployed in a province. The fact however that regular army formations like the ala singularium were formed from such elite units seems to indicate a strength of approximately 500 for both infantry and cavalry singulares. As promotions in the Roman army were as much depending on personal relations as on merit, men serving in the governor's guard could look forward to better army careers.
There is much debate on the actual service conditions enjoyed by soldiers serving in the auxilia. Recently published evidence seems to indicate that basic pay under the principate was either 1/6th part less or even equal to that of the legionaries. Auxiliaries were also included in the occasional distribution of donativa. These similar service conditions help explain why legionary soldiers were transferring freely to posts in auxiliary units. An important service condition for non-citizens enlisted in the auxilia was the grant of Roman citizenship. Generally this was awarded after 25 years of service, though on occasion grants were made during service as a reward for bravery in battle. An additional retirement grant of money for the auxilia is very likely, though the evidence available is ambiguous. The often cited difference in dimensions of the living space between the larger bases of legions and the smaller frontier forts may not have served to accentuate status differences between the legions and auxiliaries. Not only were legionary soldiers regularly stationed in the smaller forts, but the larger forts were also in part garrisoned by units of the auxilia.
The Roman emperor had several guard units at his disposal. The most important of these were the cohortes praetoriae or praetorian guard. During the reign of the Julio-Claudian dynasty the Germani custodes corporis or German bodyguard provided additional security. From the accession of Traianus the equites singulares Augusti recruited among the auxiliary cavalry formed the emperor's horse guard. The majority of these men served as guards, i.e. picked troops, rather than bodyguards directly watching over the person of the emperor. These elite forces at the emperor's immediate disposal formed the nucleus of the field armies assembled for imperial military expeditions. Smaller numbers of soldiers were selected among the guard units for personal protection duties.
Under the republic Roman generals had usually formed a guard unit named cohors praetoria after the praetorium or HQ. Under the empire such units became a privilege reserved for the emperor under whose auspicia all military operations were conducted. Augustus originally formed nine numbered cohortes praetoriae consisting of both infantry and cavalry billeted at Rome and some other Italian cities. This number was later raised to ten units and the cohorts were concentrated in a large base adjacent to Rome. Command of the praetorian guard was entrusted to one or two equestrian praefecti praetorio. Three additional cohortes urbanae with a similar structure were also present at Rome, but not under the direct control of the praetorian prefects.
A praetorian cohort consisted of approximately 500 infantrymen organised in manipuli and centuriae and under the overall command of a tribunus. This strength was doubled in the course of the first century AD. The majority of praetorians fought as heavy infantry with smaller numbers acting as light infantry lancearii and archers. Added to these foot soldiers each cohort contained a number of cavalrymen. The combined equites praetoriani numbered at least 400 men and may even have been a thousand strong. Other troopers were known as equites speculatores and served as bodyguards to the emperor. The praetorian cohort that guarded the imperial palace and accompanied the emperor in the city of Rome was known as the cohors togata. As their duties were performed within the pomerium, the sacred boundary of the city, these soldiers could not wear full armour and equipment and therefore dressed in civilian togae, though keeping their swords at hand.
Service conditions in the praetorian cohorts were better than in the legions. Pay was substantially higher and donativa were more frequent. The term of service of sixteen years compared favourably to the 20 to 25 years in the legions. Promotion opportunities were also excellent. A large part of the legionary posts as centurio was filled by former praetorian guardsmen. The cohortes praetoriae recruited originally in Italy and the older coloniae in the provinces, though at times legionaries were transferred to the guard. From the reign of Septimius Severus the transfer of picked legionaries became the usual method of filling the ranks of the praetorian guard.
The praetorian guard originally served as the backbone of field armies assembled for campaigns that involved the emperor, one of his relatives or a praefectus praetorio. Contrary to popular opinion this meant that the Rome based soldiers had a fair chance of being involved in combat either against the barbarians from across the borders or rebellious Roman army units. Despite the increase in the establishment strength of the praetorian cohorts the guards were increasingly complemented by other formations. In the course of the third century AD the cohortes praetoriae in the comitatus, the imperial field army, were regularly supplemented by mobile troops from the legio II Parthica based at Albanum in Italy. Vexillationes of elite legionaries and auxiliaries from the frontier armies joining these core formations in the imperial field army were slowly developing into separate units that were permanently attached to the imperial retinue.
The citizen guardsmen of the praetorian cohorts had their counterpart in the originally non-citizen horse guards. These consisted in the Julio-Claudian era of the Germani custodes corporis disbanded after Nero and the later equites singulares Augusti. Both these units were also known as Batavi after the tribal origin of many imperial horse guards. Members were usually recruited from the alae and cohortes equitatae, though at times men were directly recruited. The centuriones exercitatores or cavalry training officers for the imperial horse guard were however not drawn from the auxilia, but were selected from the legionary cavalry. The strength of the horse guard was approximately a thousand troopers, a number doubled by Septimius Severus. The organisation of the horse guard resembled that of the cavalry in the auxilia with turmae commanded by decuriones. An equestrian tribunus functioned as overall commander of the imperial horse guards.
- 5. The fleet
