Roman Military Forces – Imperial Period I

The legion of the Imperial period, which had developed from the legion of the Marian reforms was, tactically and administratively, a self-contained military organization. Internally, the legion was divided in many ways. Duty rosters and administrative procedures were subject to strict regulations, as well as the complicated principles of promotion within a sometimes inscrutable hierarchy (v. Domaszewski 1967). After the military reforms of Marius around 100 BC, professional soldiers who already had Roman citizenship upon entry into the force served in the legion. From then onwards, the tactical basis was the cohort, a subunit of about 500 men, which was further divided into six centuries of 80 combat troops. Each legion consisted of ten cohorts, around 5500–6400 men, with (probably only from the Flavian period) the first cohort, as the leading subunit about twice the size of the others, with c.800 men. But the actual strength of a legion would rarely have matched the nominal strength. Following H. von Petrikovits (1975), the latter can be simplified as follows (* = pure estimates).


One should not assume that all soldiers of a legion always stayed in camp in peacetime, for several hundred soldiers were assigned to special duties and were often based far outside their garrison sites.

Among the soldiers of a legion, there were some 5,000 men serving as combat troops. Most were employed as ‘common’ infantrymen, as milites gregarii. They were enrolled in the legion between 17 and 20 years of age, most having previously learned a craft. They now had every chance to advance in the complicated promotion system of the Roman army. The normal period of service for legionary soldiers was 20 years; an honourably discharged veteran then remained with his unit for a further five years (veteranus sub vexillo). After 25 years of service, he was able to return to civilian life with a pension (praemia militiae), which consisted of cash and/or a land grant. At least in theory: in the early Imperial period there were repeated problems of mutiny, because soldiers were not released on time, but were forced to serve for longer. But it was also possible to continue to serve as a volunteer (voluntarius). The veteran with regular service proudly stressed that he was a missus honesta missione, ‘dismissed with an honourable discharge’. There were also premature retirements as a result of disease (missio causaria) or even dishonourable dismissal from the army (missio ignominiosa).

The common soldiers performed their daily duties within one of the ten cohorts. With the exception of the first, each cohort in turn consisted of six centuries (‘hundreds’) each commanded by a centurion. Eighty common soldiers served in every century; the remaining 20 were composed of specialists such as craftsmen, doctors, soldiers with administrative tasks, legionary riders, and so on. Two centuries formed a maniple in the 2nd and 3rd centuries, by then only important administratively and for the cohesion of the soldiers. Thus, each cohort had three maniples, the legion 30 of them; as the types of armament were named in the same way as in the early days of the Republic – triarii, principes and hastati – the maniples carried the same names. Each maniple had its own standard (signum).

Each legion possessed 120 horsemen (equites legionis). They probably served primarily as dispatch riders and messengers. In battle, they fought together with the legion’s auxiliary cavalry, separate from the legion itself.

Legionary commanders and military tribunes

The commander of a legion, the legatus Augusti pro praetore legionis, was appointed by the emperor. If only one legion was based in a province, he always also fulfilled the office of governor. He thus administered both the military and the civil administration of a province. The next highest-ranking officer after the legionary legate was a senatorial military tribune, the tribunus militum laticlavius. Since this officer, by birth or by the favour of the emperor, was a contender for the post of quaestor (minimum age requirement 25 years) and thus of senatorial rank, he was allowed to wear a broad purple border (latus clavus) on his tunic. He was very young, usually only 18 to 20 years old. Nevertheless, he was the highest ranking officer after the legionary commander. Lacking a fixed command role, he was a deputy in the event of war. Such a senatorial military tribune often had a brilliant career in the military and the administration before him.

From the second tier of Roman nobility came the equestrian military tribunes who, because of the narrow purple stripe on their tunics, were called tribuni militum angusticlavii. There were five of them in a legion. They too were often at the beginning of great careers in the procuratorial service, for which service as an equestrian officer was a precursor. These equestrian tribunes spent most of their time in administrative tasks. Sometimes they also led a cohort, commanded some special detachments, or helped the commander in other important tasks.

Legionary prefects

The military tribunes were serving in the army for only a relatively short term, quickly continuing their career path of mixed military and civilian postings. Accordingly, they cannot really be called professional soldiers. Their practical military experience was probably not very wide-ranging in most cases. However, there was at least one officer in the command structure of each legion who had worked his way up from recruit as a professional soldier in the army: the praefectus castrorum (legionis). In terms of rank, he was below the senatorial tribune, but above the equestrian tribunes. He belonged to the equestrian order. The area of responsibility of legionary prefects could be described as base commander and logistics officer. He was responsible for the camp and the associated legionary territory, also controlling the entire roster including guard duties. In the period from around the Emperor Domitian (AD 81–96) to Emperor Septimius Severus (AD 193–211), they were called praefectus castrorum with the occasional addition of legionis, but afterwards just praefectus legionis. This title they retained even when, after Gallienus (AD 253–68), they commanded the entire legion, because there were now no longer any senatorial officers in the legion, although they had already done so earlier in the absence of the legatus legionis and tribunus militum laticlavius. All the above-mentioned officers had their own administrations.


Centurions can be described as the military backbone of a legion. These officers – equivalent to the modern rank of captain – were professional soldiers through and through. They usually commanded the centuries, administratively and tactically the smallest subunits of a legion. Ultimately, they were responsible for ensuring that, in the event of war, military operations were performed successfully. Therefore, they had to pay particular attention to the maintenance of discipline in troops in time of peace. As part of this duty they enforced the commands of the legion’s high command with the ordinary soldiers. They also monitored the daily routine of the combat troops, drill as well as manoeuvres, preparation of parades, and inspections, and likewise the introduction of new recruits or the cleaning of accommodation. In battle, they were at the forefront. In more peaceful times, centurions also commanded work details of legionaries in mines, quarries, or brick kilns, generally on imperial and state possessions. Sometimes they were temporarily employed as commanders of building detachments, such as for the erection of forts or fort baths for auxiliary units.

As a sign of his disciplinary authority, the centurion carried a vine staff and wore greaves, a transverse crest, and a gold ring although, after Septimius Severus, non-commissioned officers (principales) also had these.

Most centurions had been promoted from the lower ranks and only achieved their position after 13 to 20 years of service. The evocati, veterans who voluntarily returned to the colours, also had good career opportunities, most of them being promoted directly to centurions.

According to the number of centuries, there were 60 centurions in a legion. They were allotted to the cohorts and again to the maniples. Since each of the three maniples in a cohort comprised two centuries, each had two centurions. Like military tribunes, these were divided between a prior (more experienced) and a posterior (less experienced). In pithy military language, therefore, the six centurions in each cohort were named according to the old manipular terms pilus prior and pilus posterior, princeps prior and princeps posterior, and hastatus prior and hastatus posterior. The commander of the entire cohort was the pilus prior. The cohorts in turn were numbered from the first (prima) to the tenth (decima), so that each centurion could be precisely identified. Of course, the difference in rank between the centurions of the second to the tenth cohort was not particularly serious. Only the centurions of the first cohort (primi ordines) occupied a special position.

This was because the entire first cohort significantly differed from the other nine in many ways. It was almost twice as strong, although it had only six centuries; but it had another 400 soldiers: mainly the ordinary soldiers and NCOs under the legionary prefect, as well as soldiers on long-term duty outside the garrison base. The most senior centurion, the primus pilus, also led two centuries (Dobson 1978). This position, apparently only held for one year, could even lead to elevation to equestrian rank. The primus pilus was first and foremost the centurions’ spokesman with the commander.

After his year in office, this officer (with a minimum age of 50 years) either retired and played a prominent role in his home region or was next promoted to praefectus castrorum legionis. With that, his military career was finally over. However, if – after the primipilate – he went to the capital city of Rome and served as a tribune in the fire service (cohortes vigilum), city cohorts (cohortes urbanae) or in the imperial bodyguard (cohortes praetoriae) as tribune, he could return to the legion as primipilus iterum (‘primus pilus returning for a second time’); then he occupied a special place on the staff of the legion commander and ranked immediately below the senatorial military tribune. Such men had a good chance of being entrusted with a post as an equestrian in command of a legion in Egypt or even with an imperial procuratorship, i.e. a high and well-paid administrative office. The majority of the men certainly never got so far and were discharged as centurions. However, during his active career, a centurion did not have to remain with the same legion. The reasons for transfers differed; they might be found on a personal level or could be a result of patronage. Technical specialists also often changed postings.

Principales and immunes

One group of soldiers, entrusted with special tasks, ranked above ordinary soldiers but below centurions. They were called immunes (exempt from fatigues, corresponding roughly to modern privates) and principales on a slightly higher level (ranking similar to modern NCOs). Since the two terms (the latter first appeared at the beginning of the 2nd century) were not always used unambiguously, it is often not possible to specify the exact rank of individual posts (it is even more difficult to determine possible career structures). In addition, it is to be expected that one or other was subjected to fluctuations in status over time, i.e. it would increase or decrease in status. These included the weapons orderlies (armorum custodes) assigned to each century. Although they were still among the immunes around AD 180, a few decades later they were definitely principales. The same was probably true for scribes and copyists, the librarii and exacti, who, if they worked in the office of the governor, belonged to the sesquiplicarii, especially in the 3rd century AD. The main difference between the immunes and the principales lay in their pay and that the principales were exempt from the duties of ordinary soldiers by virtue of rank, while it was granted to the immunes specifically to exercise their special duties. In turn, principales were further graded according to whether they received one-and-a-half times or twice the basic pay: whether one was a sesquiplicarius or dupl(ic)arius.

Whoever had achieved this ‘non-commissioned officer rank’, enjoyed further, additional privileges, such as more spacious accommodation and communal areas in camp. Nevertheless, there were differences even within this rank. For daily routine, it was important whether a principalis belonged to the combat troops, and thus served in a century (such as, for example, the optio, tubicen, and signifer) or on the staff. Among staff positions, another two groups are distinguishable: office posts (officiales), designated according to the rank of their officers, and tactical posts which – seconded from the centuries – served on the staff of the governor, like the eagle-bearer (aquilifer). He carried the standard of the whole legion, topped with a golden eagle. Apparently always a member of the first cohort, he was the highest-ranking NCO not in an office. This post was not always achieved after a long period of service. It seems that sometimes eagle-bearers (there were possibly two per legion) had previously been standard-bearers (signiferi) and were later even promoted to centurio.

The head of the administrative staff was called the cornicularius, both in military offices as well as in the civil administration of the relevant officium of the governor. He oversaw the lower-ranking clerical staff (e.g. the exacti and librarii), and even undertook clerical work himself, especially in the offices of the lower staff officers. If he worked for the legionary commander and governor, he could hope for promotion to centurion. The beneficiarii consularis reported directly to the governor. They were used for special tasks, for instance the financial and economic control along roads (Schallmayer 1990 and 1992). The signifer carried the signum, a standard with discs attached (Alexandrescu 2010). Apparently, two such standard-bearers belonged to each maniple. In addition, this rank was responsible for monitoring the cohort funds and the savings of the soldiers; he was also responsible for the burial fund. Until the 1st century AD, the frumentarii provided their units with grain; accordingly, they derived their name from it (frumentum = grain). Later, however, they were used for various tasks: as messengers and as a supervisory officers.

The optio commanded the century if the centurion was ill or otherwise incapacitated, also helping him with management tasks. As a kind of ‘sergeant’, he was waiting to join the centurionate. Each maniple had a tubicen, a tuba player (or other brass musician), as well as a cornicen (horn player) (Alexandrescu 2010).

Roman soldiers were given regular pay, which was differentiated according to the rank and service grade of the troops, in return for their by no means easy duties. However, there were deductions from pay for weapons and equipment. In addition, a part of the pay was withheld, to be paid out as a bonus upon honourable discharge from the army – here, something approximating one of the principles of modern national insurance was anticipated. A share of booty during war and one-off donatives upon the accession of a new emperor could on occasion supplement pay.

The total number of legions during the Imperial period repeatedly fluctuated due to losses and new formations, but there were never more than 28 units at the same time.