Roman Military Forces – Imperial Period II

Warrior auxiliary cohort of Batavia , the second half of the 1st century AD.

Batavian auxiliary cavalry – Pablo Outeiral

Auxiliaries (auxilia)

About the same strength as the legions, there was a second part of the Roman army, the so-called auxilia or auxiliary troops (Kraft 1951; Spaul 1994 and 2000). In terms of strength, pay, and legal status, these were ‘second-rate’ units. From the mid-1st century BC, auxiliary troops were mainly used for frontier protection, but when in combat, they operated alongside legionary troops. In the Imperial period, auxiliaries provided all of the cavalry (the few mounted legionaries were insignificant in impact). Those free imperial residents from the provinces who did not initially possess Roman citizenship, the so-called peregrini (= strangers) could enrol in the auxilia. If they had completed 25 years of service and if they had done nothing wrong, then they could get the coveted Roman citizenship.

In the Republic, auxiliary troops were tightly bound to the legions in organization, garrison, and use. They were also housed with them – albeit spatially separated – in the same camps. After the Augustan period, their position changed: they became increasingly independent units and had, from the Claudian period onwards, their own camps – the auxiliary forts.


The auxilia were divided between cavalry and infantry units into alae and cohortes, 500 (quingenaria) or 1,000 (milliaria) men strong. Frequently, there were also mixed units, so-called cohortes equitatae.

A cohors quingenaria peditata consisted of six centuries, each of 80 infantrymen under the command of a centurion; including the staff, it was about 500 men strong. Their commander, the praefectus or praepositus cohortis, usually came from the equestrian order.

A cohors milliaria peditata consisted of ten centuries, each of 80 infantrymen under the command of a centurion; with staff, it was perhaps about 1,000 men strong. Their commander, the tribunus cohortis, usually came from the equestrian order.

The cohors quingenaria equitata was a mixture of infantry and cavalry. It consisted of six centuries of 80 men under the command of a centurion. To these were added six turmae of cavalry; it was probably about 500 men strong. Their commander, the praefectus cohortis, came from the equestrian order.

A cohors milliaria equitata was also a mixed unit of infantry and cavalry. It consisted of ten centuries of 80 men commanded by a centurion. In addition, there were ten turmae of 24 cavalrymen, so together with staff the unit was over 1,000 men strong. Their commander, the tribunus cohortis, came from the equestrian order.

An ala quingenaria consisted of 480 horsemen, who were divided into 16 turmae of 30, the leader of the turma being the decurio (not to be confused with that post on a city council). The ala milliaria consisted of 1,008 horsemen, who were divided into 24 turmae of 42 men. The commander of an ala quingenaria, like the ala milliaria, was always a praefectus alae, drawn from the equestrian order.

Commanders of auxiliary cohorts were appointed by the governor of the province in which they were stationed. In contrast, the commanders of alae, the praefecti alae, were always appointed directly by the emperor in Rome. They completed their military service after the time of Emperor Claudius (AD 41–54) in three tiers, namely in the career of the tres militiae which corresponded to three grades:

1.Prefect of a cohors quingenaria;

2.Military tribune in a legion (tribunus militum legionis angusticlavius ) or as a tribunus cohortis milliariae;

3.praefectus alae quingenariae.

Then, around the middle of the 2nd century AD, a fourth tier was added for the command of an ala milliaria with the militia quarta, only rarely achieved, in the quarta militia equestris, namely the rank of praefectus ala milliariae. This was the highest military rank that an equestrian could attain, very often paving the way for a procuratorship, managing the financial administration of a province.


While the Romans initially drafted auxiliary troops from the able-bodied young men of newly conquered territories and based them away from the recruitment area, over time the majority of units increasingly came from the hinterland of their base, with only a few exceptions. Often, auxiliary units were named after the tribe from which they had been originally recruited, for example the cohors Raetorum. However, if such a Raetian cohort was posted in Britain for a long time, for example, the name became merely traditional, because over time the unit came to be formed mainly from provincials, in other words from Britons. In contrast, there were special forces with indigenous weapons or special abilities that were extremely important to the Roman army. These included, for example, the Batavians from the Lower Rhine. They were not only first-class fighters on foot and on horseback, but they could also do what others apparently could not, crossing water in full armour with their horses. They evidently trained in these skills from early youth. So, along with oriental archers (mainly from Syria), they belonged to those auxiliaries who were not just placed in an area and then raised recruits from around their garrison town, but also kept recruiting from their original recruitment region. Moreover, the officers of these units were aristocrats from the same tribes. There were some auxiliary units (cohortes civium Romanorum) in which Roman citizens served too, while cohortes classicae were a relic of the Civil War, as they were infantry units formed after 27 BC from marines left over after the battle of Actium.

Auxiliary units were also named after the emperor, or an officer under whom they were founded, for example the ala Flavia or ala Indiana (after the Treveran noble Julius Indus). There were also numerous honorary names if the whole unit had performed well, for example pia fidelis (reliable, loyal) or victrix (victorious). Special armament was also occasionally added to the title of the unit, e.g. sagittariorum (archers) or scutata, meaning the troops fought like a legion, with the scutum, the rectangular shield.

Up to the mid-1st century, the auxilia were still closely tied to the legions, tactically as well as spatially and in their accommodation. Only with the formation of stable frontiers on the limites and ripae did the auxilia win more independence and were posted further away from the legions to man the frontiers. (However, an interesting exception is the legionary fortress at Bonn where an additional ala and a cohort were based alongside legio I Minervia until the 3rd century AD.)


With his honourable discharge (honesta missio) after 25 or more years of service, better times began for the auxiliary soldier. The savings that had been withheld for him in the unit coffers and that had built up over the years were paid out to him in cash; in addition he now finally received Roman citizenship, extending to his wife and his descendants. Through the ‘law of legal marriage’ (ius conubii), also awarded to him at that time, an existing or proposed matrimonial alliance with a ‘foreign’ woman was now fully recognized before Roman law. So now he could live in a family recognized by Roman law for the first time! The citizenship acquired via service in the auxiliary troops helped millions of provincials to become the legal and social equals of Roman citizen who had moved to the provinces after the conquest. These privileges, of which the most important were citizenship and legal marriage, were so important that it is no wonder that many soldiers wanted them confirmed in duplicate. These documents had the form of bronze plaques, so-called ‘military diplomas’. They contained the text of the imperial decree of the honourable discharge of this soldier from military service, as well as the date, the names and seals of a number of witnesses, the personal data of each recipient and his wife and children, his direct and indirect superiors, as well as a list of all troops affected in the relevant province, an extremely important source for Roman military history (Eck and Wolff 1986; Eck 1997 and 2010a; Pferdehirt 2004). The original of this document was posted publicly at the Temple of Minerva in Rome, so the recipient only received a copy.


From the late 1st century, the so-called numeri appeared, ranking below the auxilia (Reuter 1999a). These consisted of 100- to 200-strong guard and reconnaissance units, stationed along fixed lengths of frontier. If such units were mounted, they were called exploratio. They were under the command of a praepositus numeri, often a centurion seconded from a legion. Numeri were used in addition to the alae and cohorts for frontiers or to explore and secure areas beyond the frontiers, particularly in more remote forest regions, like the Taunus and Odenwald in Germany or on the Dacian limes. In contrast to other auxilia, they always seem to have been used in the same province, and always in the same frontier sector; they were not, for example, sent on campaign. This proves that, as local scouts, they were familiar with a particular section of frontier. The names of the numeri were often formed from the nations from which they had been raised, as with, for example, the diverse numeri Brittonum or numeri Palmyrenum. In order to identify a specific unit among the various numeri Brittonum in Upper Germany, the name of a river or a source in their territory was added to the name of the unit. An example is the numerus Brittonum Elantiensium, named after the River Elz, a tributary of the Neckar.


The emperor and the provincial governors were protected by guard units (Fig. 8), whose soldiers were detached from the regular army (Busch 2010). These cohortes praetoriae, introduced by Augustus, guarded the emperor and his family and went to war with the emperor. They originally numbered nine cohorts of 1,000 men, each made up of 900 infantry and 100 cavalry. From the time of Tiberius (AD 14–37), they were based in the Praetorian Camp in Rome and were for a long time the strongest military power in and around Rome.

Augustus also introduced a mounted guard of Germans, the Germani corporis custodes, although their strength and organization are not known. This unit was disbanded under Galba. Only under Trajan was there a mounted guard again, the equites singulares Augusti. Furthermore, there were other special units recruited from troops in.

Rome, such as the speculatores Augusti, evocati, statores, frumentarii, and speculatores legionis. In addition, soldiers of the Imperial fleets from Misenum and Ravenna were reassigned to operate the awnings in the circus and in the theatres.

To these regular military units were added the cohortes urbanae, the city soldiers, and the cohortes vigilum, a kind of military fire service and night watch.

Not until Septimius Severus (AD 193–211) did the military presence of the Praetorians in and around Rome become much stronger. First, the Praetorians were temporarily disbanded and a legion, legio II Parthica, was stationed in Albano, near Rome; then the theoretical strength of the Guard was increased considerably.

In the praetorian frontier provinces without legionary garrisons, there was a ‘guard’ for the governor consisting of around 250 cavalrymen and infantrymen, the equites and pedites singulares, which consisted of selected men from the provincial army. In the provinces with legionary garrisons, the governor’s guard consisted of selected legionaries.

Troops and paramilitary units in Rome during the Imperial period (after Busch 2010)

Theoretical strength of troops in Rome during the Imperial period (after Busch 2010)


From the early Imperial period, there were also local militia units in the provinces: iuniores or iuventus associations are testified, without details of their number, structure and military armament being known. They gained greater importance, especially in the turbulent times of the 3rd century, and were then more frequently attested epigraphically. Thus the inscription on the Augsburg victory altar names populares, unspecified militiamen, as jointly responsible for the Roman victory of 24th/25th April 260 over the Semnones/Iuthungi (Bakker 1993).

The so-called ‘military reform’ of the 4th century AD

With few exceptions, the Roman army – legionary and auxiliary troops – was based along the frontiers in permanent legionary camps, as well as ala, cohort, and numerus forts, from the time of Augustus. Apart from the Praetorians and bodyguards of the emperor, Italy had no military presence; only from the time of Septimius Severus (193–211) was there a legion south of Rome, in Albano. This meant that the military high command had no significant mobile reserves present in the interior of the empire to deploy alongside the frontier army.

In this system, concentrations of military force, such as were necessary for field campaigns or in the defence of larger attacks, were difficult and time-consuming to accomplish, as ad hoc army groups (vexillationes) had to be assembled from the various provincial armies to undertake specific combat missions (Saxer 1967). If the task was completed, the individual components of these vexillationes returned to their regular units. While this approach generally worked until the 3rd century, or the practical-minded Romans would not have kept it, during the 4th century the weaknesses of it became increasingly obvious. At a time when there were often multiple simultaneous external attacks on the Rhine and Danube frontiers by Germans, Sarmatians, and other opponents, and in the East by the Parthians and then by the Persians, this system simply collapsed. The withdrawal of troops from a momentarily seemingly less-endangered western frontier for field campaigns in the East now meant that Germanic opponents recognized the weakness and regularly took the opportunity to attack the frontier provinces. In addition, the wars between rival claimants to the throne often meant that the best troops of the Empire fought each other, often to the point of almost total annihilation, from the time of Septimius Severus onwards.

The first large-scale measures to reform the Roman frontier defensive system and get this problem under control can be detected under Gallienus (AD 253–68). Since the effectiveness of the Persians and Germans was probably that they were mostly mounted, a cavalry formation – which was mobile and stationed in Milan – was set up by the emperor from new units to protect Italy and Rome from threat. They could even catch an opponent if they had already broken through the frontier defences and had penetrated far into the hinterland. In late antiquity, this system was perfected with a functional division of the army (border guard troops in garrisons on the frontier, mobile task forces in the hinterland) and introduced throughout the empire. The older view of this dual system as a new introduction in the context of military reform under Diocletian and Constantine is being increasingly called into question by research. Thus, for example, K. Strobel (2007) and Y. Le Bohec (2010) emphasize the continuous development of the comitatensian or mobile army from the mid-Imperial period onwards. However, fundamental changes in the Roman army arrived in the 4th and 5th centuries AD, but are difficult to reconstruct in detail. In general, the Late Roman military system is much less transparent than those of the Republic and the early to mid-Imperial period. Due to their scarcity, written sources generally play a much smaller role for the military history of late antiquity than for the preceding period; the number of inscriptions generally decreased radically, in other words epigraphic evidence with military implications – which previously constituted the most important source for the organization and distribution of Roman troops – disappeared almost completely. Here, even the text originating in the early 5th century AD, the ‘directory of all offices, both civilian and military in the western parts of the empire’ (Notitia dignitatum omnium, tam civilium quam militarium, in partibus Occidentis), known for short as the Notitia Dignitatum, is hardly a substitute. But at least the units of the frontier army and weapon factories are listed in it.

From at least the reign of Constantine I (AD 306–24), the Roman army was divided in two: there were now a frontier army permanently based in frontier forts, the limitanei or riparienses, and a mobile army operating in the hinterland, the comitatenses. The frontier units were largely maintained along earlier lines, in terms of formation and tactical designation (legions, alae, and cohorts), but their theoretical strength was significantly lower than that of the same units of the early and mid-Imperial period. This can be seen from fort sizes, compared to the early and middle Imperial period.

According to the Notitia Dignitatum, the military command of the frontier troops of the Western Empire lay in the hands of the highest-ranking soldier in this part of the Empire, who held various titles and had diverse areas of competence over the course of time. He was called magister militum [utriusque militiae] (‘master of the army’ or ‘master of all military branches’), or at the Imperial court magister peditum praesentalis (‘master of infantry present [at court]’); after c. AD 370 he was addressed as Illustrious One (vir illustris); the dux limitis (frontier sector commander) reported to him. Occasionally, this post was filled by one person for several provinces, like the military commander of the two Raetian provinces. He bore the title of dux provinciae Raetiae primae et secundae (General of the First and Second Raetian Provinces) with the rank of Admirable One (vir spectabilis).

Diagram showing the hierarchy of the Roman fleets of the early and middle Imperial period (after Pferdehirt 1995)

The Roman navy

The Roman fleet of the Imperial period, like the auxiliary troops of the Roman army, took as recruits free, but legally and socially underprivileged, peregrine residents of the Empire who did not possess Roman citizenship. However, they could gain the coveted Roman citizenship for themselves and their families through military service. However, since naval service was considered more difficult than that of the forces on land and since, with a total of 26–28 years, it was also longer than the 25 years of service for the legions and auxiliary troops, those entering the fleet gained the special privilege of Latin citizenship for themselves from the start – but not for their wives and children. Although this did not offer all the privileges of full Roman citizenship, it was a huge leap forward in legal status for a peregrine provincial. A fleet soldier received full Roman citizenship for himself and his family only upon honourable discharge, after having completed the whole term of service.

Of course, this rule did not apply to the officers who, from the outset, had to possess Roman citizenship. The supreme commander of a fleet, the praefectus classis, came from the equestrian order.

In organization and jurisdiction, the Roman fleet was divided into two parts, with a clear distinction apparent between the sailors and the marines. This went so far that every ship had two commanders, one nautical and one military. The nautical commander bore the title nauarchus on larger vessels, on smaller ships trierarchus; these designations were quite obviously adopted from the eastern Hellenistic area, albeit with a certain change of meaning.

A particular theme in Roman military archaeology – the iconographic and textual sources, and especially the original remains of Roman warships – has recently led to the development of a separate direction in research: the study of ancient navigation. The discovery of numerous shipwrecks in the Mediterranean, but also of warships used on the Rhine and Danube, like the ships from Mainz and Oberstimm, has brought further advances here. In particular, the Museum for Ancient Navigation (Museum für antike Schifffahrt) in Mainz (affiliated to the nearby Römisch-Germanisches Zentralmuseum) has had a major impact in this as an international research institute.

Military crews were equivalent to the centuries of the land army. These consisted of about 80-strong combat units under the command of a centurion. Therefore, there was a marine captain with the rank of centurio classicus. Above all, the sailors were often specialized, there being helmsmen, navigators, a man who was on the lookout in the bow, warning of shoals, a coxswain for the rowers, loadmaster, doctors, and others.