The Roman Army during the Pax Romana (27 BCE – 180 CE) was structured into legions, auxiliary units, and naval forces, with soldiers drawn from both Roman citizens and non-citizens. Under reforms initiated by Augustus, the Roman military became a standing army, featuring standardized recruitment, training, and equipment, along with fixed terms of service and regular pay. The legions, composed of heavy infantry, were the backbone of the army, while auxiliary units provided specialized roles, such as cavalry, archers, and engineers. These forces both secured the empire's frontiers and contributed to infrastructure development.
Command, control and organization
The Roman Emperor was the sole Imperator (Commander-in-Chief). He commanded his troops via the Legati Augusti pro praetore who served as either as a general or a governor of an imperial province.
Legion The legions were the principal force during the Pax Romana. Each legion had a numerical designation that was permanent. But due to the earlier civil wars these were sometimes duplicated, thus each legion had an individual name. These names could originated: from the name of a province where it was originally stationed, from an Emperor's dynastic name, from a god, from an aspect of its earned renown, from its formation as the ‘twin’ of an earlier legion or from some visible element of equipment.
The legions of the Roman Army did not always operate as complete formations. Temporary task forces, known as vexillationes (derived from vexillum, meaning "flag"), were frequently assembled on an ad hoc basis by detaching troops from legions to address specific crises or undertake offensive missions. These detachments often included auxiliary troops and varied in size, but a typical vexillation might consist of approximately 1,000 infantrymen and 500 cavalrymen. From 13 BCE to the mid-1st century CE, legionary veterans were transferred to a unit called the vexillum veteranorum after completing 16 years of service. They served an additional four years in this unit, which had a strength of about 500 men and its own officers. These units could be attached to their parent legion or deployed independently. Under Emperor Vespasian, the total term of service for legionaries was extended to 25 years.
The internal organization of a legion, while subject to minor variations over time or between different legions, generally adhered to a consistent structure. A legion's infantry strength ranged from approximately 4,800 to 6,200 milites legionis (legionary soldiers). Each legion was divided into centuriae (centuries), typically consisting of 80 men, though some might number 100. Each century was commanded by a centurio, assisted by junior officers, including the signifer (standard-bearer) or the aquilifer (eagle-bearer) in the First Century of the First Cohort, the optio (the centurion's deputy), the cornicen and bucinator (trumpeters), and the tesserarius (officer of the watchword).
Specialized roles existed within the legion, such as the custos armorum (a soldier responsible for maintaining equipment) and the beneficiarius (a soldier or junior officer exempt from fatigue duties, often serving as a bodyguard or undertaking special tasks). The antesignani ("those before the standard") and postsignani ("those behind the standard"), terms dating back to the Consular period, continued to appear in sources during the Imperial era, referring to picked men fighting in front of or behind the standards.
Six centuries formed a cohors, a battalion of 480–600 men. Cohorts could also be subdivided into three manipuli, each consisting of two centuries and numbering 160–200 men. A legion typically comprised ten cohorts, though from the Flavian period onward, the First Cohort was double the size of the others, with 800–1,000 men organized into five double-sized centuries. This cohort was led by the primus pilus ("first javelin"), the most senior centurion in the legion. By the late 1st century CE, however, the tribunus laticlavius, a young aristocrat from a senatorial family, commanded the First Cohort. This tribune was one of six tribunes assisting the legionary commander, primarily in administrative roles; the other five were tribuni augusticlavi, from the equestrian social class.
The post of legionary commander was, except in Egypt, held by the legionary legate, a man of senatorial rank in his mid-thirties who had already held the praetorship in Rome. In provinces where there was only one legion, the posts of legionary legate and provincial governor were combined. The legate would expect to serve for three years and would, if married, normally be accompanied by his wife and family. As a man in his first or only legionary command would have little direct experience of military matters, he would rely heavily on the advice and experience of the senior centurion (primus pilus).
Below the legate, the officer with the most extensive military experience was the praefectus castrorum (camp prefect), usually a former primus pilus. The camp prefect oversaw all practical aspects of the legion's operations and assumed command in the absence of the legate or senior tribune. Theoretically they were appointed to specific legionary bases rather than individual legions, so that where two legions were brigaded together there was only one camp prefect. Each legion also included an integral cavalry unit of about 120 men, used primarily for reconnaissance and courier duties. These cavalrymen were organized into four turmae (squadrons) of 32 men each, with each turma commanded by a decurio. Additionally, each cohort of the legion was supported by approximately 120 calones (servants).
Praetorian guard
The Praetorian guard acted as the bodyguards of the emperor. The Praetorian guards were recruited from good family in the central Italian regions of Latium, Etruria and Umbria. During the reign of Augustus they were also recruited from the north of Italy. Throughout the first century AD, provincials from distant regions such as Thrace, Pannonia and Germania major were recruited into the cohorts too. Similar to legionaries, Praetorian guardsmen — whether milites (infantrymen), equites (cavalrymen) or speculatores (elite troops) — had to be Roman citizens. According to Herodian, Praetorians had to be tall. He describes them as megistoi, a Greek word meaning 'the biggest' or 'the greatest' (physically). Recruits typically enlisted between the ages of 17 and 20. Following the setting of the length of praetorian service at 12 years by Augustus in 13 BC, this was increased to 16 years 18 years later. Nevertheless, examples of praetorians retiring after 16 years are rare, even during the Augustan period or the reign of his successor, Tiberius (AD 14–37). Under Augustus there were nine praetorian cohorts each 500 strong. This number was increased to twelve, probably by Caligula,’ and Vitellius in the panic of the Civil War enrolled sixteen from legions loyal to himself, but Vespasian, with typical frugality, reduced the number once more to nine and the size to 500 each.
cohortes urbanae
The cohortes urbanae (urban cohorts) were a specialized paramilitary force in ancient Rome, primarily responsible for maintaining public order and security within the city of Rome and other major urban centers of the Roman Empire. They were established by Emperor Augustus in the late 1st century BCE as part of his efforts to reform and centralize the Roman military and administrative systems. The cohortes urbanae were distinct from both the regular legions of the army and the Praetorian Guard, occupying a unique role as an urban peacekeeping force.
Cavalry The Roman heavy cavalry during the pax romana was a mix of units drawn from both Roman citizens and auxiliary forces recruited from allied or subjugated peoples. The primary heavy cavalry units included: Alae (Auxiliary Cavalry Wings), Equites Legionis (Legionary Cavalry) and Equites Cataphractarii (Armored Horsemen). Like the infantry cohorts, there were two sizes of ala, the ala quingenaria, (500 troops) and the ala milliaria (1000 troops). Each ala quingenaria consisted of sixteen squadrons (turmae) of 30 men. The ala milliaria made up of twenty-four turmae, which also consisted of 30 men. The turmae, like the centuries, will have been subdivided into contubernia, but the size of these is not known. An ala was commanded by a Praefectus Alae. This officer was often an equestrian-status Roman citizen with broad military experience. Below the Praefectus was the decurio and his second-in-command the duplicarius, in charge of a turma.
Immunes
Although the roman soldier was trained to perform most of the special task like bridge building or creating defence works there were also specialized units and individuals within the Roman military who were specifically trained and designated for certain technical or support roles. These specialists, often referred to as immunes, were exempt from regular duties such as digging ditches or standing guard, allowing them to focus on their particular expertise. While the average Roman soldier was capable of constructing fortifications, siege engines, or temporary bridges as part of their general training, the immunes provided the technical knowledge and oversight required for more complex projects.
Among the large group of immunes were Engineers (Architecti) and Artifices (Fabri): responsible for advanced construction tasks, such as building permanent bridges, aqueducts, and fortifications. They also designed and operated siege engines like ballistae, onagers, and battering rams during sieges. Their expertise was importante for large-scale engineering projects that went beyond the basic skills of the regular soldiers. Surveyors (Agrimensores) who were tasked with measuring and planning construction sites, including camps, roads, and fortifications. They used tools like the groma and chorobates to ensure precision in aligning roads or laying out military camps. Artillery Specialists (Ballistarii) who operated and maintained the army's siege engines, such as catapults and ballistae. Their training allowed them to calculate trajectories and ensure the proper functioning of the machinery during combat. Additional there were also Blacksmiths (Ferrarii) and Armorers (Armorum Custodes), Shipbuilders and Sailors (Classiarii): Clerks (Beneficiarii) and Administrators (Librarii), Animal Handlers (Muliones) and Medical Staff (Medici).
Auxilia
Auxiliary infantry regiments were designated as cohorts and were organised in a manner analogous to the organisation of the cohorts that constituted the legions. The majority of these regiments (cohortes quingenaria) comprised six centuries of 80 men each, totalling 480. However, some regiments (cohortes milliaria) consisted of ten centuries of 80 men, resulting in a total of 800, which was equivalent to the five double-strength centuries of the first cohort in the legion. As was the case with legionary centuries, the auxiliary centuries were subdivided into contubernia of 8 men each. Auxiliary units were normally commanded by men of equestrian rank, although their place was sometimes taken by centurions seconded from the legions. Cohortes quingenariae were commanded by a praefectus while cohortes milliariae were under the command of a tribune. Under the overall command of the prefectus or tribune in charge of each auxiliary unit were the centurions.
Cohors equitata The cohortes equitatae were a type of mixed auxiliary unit in the Roman army that combined infantry (pedites) and cavalry (equites) within a single formation. Unlike the legions, which consisted solely of heavily armed Roman infantry, the cohortes equitatae were designed to provide greater tactical flexibility by combining the strengths of both infantry and cavalry in a single unit. A standard cohors equitata was typically organized into two primary types: