The organisation of the Roman Army under the early Empire

To appreciate how Rome’s armies worked under the early empire, it is necessary to understand two defining attributes of Roman society: firstly, citizenship and secondly, the emperor’s relationship with his armies.

First, Roman citizenship. It was possible to become a citizen, you did not have to be born one. This approach to identity played a vital role in determining the growth and character of the Empire, though it lost much of its significance when in AD 212, the Emperor Caracalla passed an edict (the Constitutio Antoniniana) extending citizenship to all free born subjects within his dominions. The distinction between citizens (those with a privileged legal status) and non-citizens was important, but there were recognised ways in which people could acquire citizenship. One of these was military service.

Second, the emperor’s relationship with his armies was a central theme of Roman history. Soldiers swore their military oath to the emperor and served his interests. That was their primary role; the defence of the provincial subjects was not a priority unless it served the emperor’s wishes. Much of the Empire’s tax revenue went to paying for this military.

The distinction between citizen and non-citizen that ran throughout the Empire also informed the organisation of Rome’s armies. In theory at least, only citizens could serve in the emperor’s Praetorian Guard (his bodyguard), and the legions. Yet more than half of Rome’s land forces consisted of auxiliaries, a term seemingly coming from the notion of ‘helping forces’. To join these one did not have to be a citizen.


The legions (formations of citizen infantry) were between 5,000 and 6,000 strong. During the first century of conquest the Empire had approximately 30 legions (sing. legio, pl. legiones), each strategically located. Each legion was divided into 10 cohorts (sing. cohors, pl. cohortes), the first larger than the rest. Each cohort was in turn subdivided into centuries (sing. centuria, pl. centuriae) which in practice often fell short of a literal century of 100 men. Most were probably about 80 strong.

A legion was commanded by a legionary legate, drawn from the highest echelons of society (the Senatorial Class). The legate would not have been a career soldier. He would have seen his command appointment as a step in a wider career that involved periods in diverse civil and military roles. He was in turn supported by tribunes, and centurions (sing. centurio, pl. centuriones). Many centurions came up through the ranks. They had their own complex hierarchy with the primus pilus being the senior centurion of the legion. Distinguished by their vitis, or cane of office, the centurions were generally career soldiers. Each centurion had a second in command or optio, whose badge of office was a long cane with a knob at one end.

It is often imagined that because they were formed of Roman citizens, the legions that came to Britain were full of Italians – they were not. Most were composed of soldiers drawn from citizen communities outside Italy. Quite a number, for example, came from Gaul and Germany. The crucial issue was not place of birth, but citizenship.


Auxiliaries served in a range of smaller formations commanded by prefects or tribunes. There were cavalry regiments (sing. ala, pl. alae) which were subdivided into troops (sing. turma, pl. turmae) under not centurions, but decurions (sing. decurio, pl. decuriones). Infantry units (sing. cohors peditata, pl. cohortes peditatae), were organised much like infantry cohorts, and mixed units of cavalry and infantry. A mixed unit or cohors equitata (pl. cohortes equitatae), was an adaptable formation well suited to a range of frontier tasks, but their cavalrymen did not enjoy the same status as their counterparts in the alae. Cohorts and alae came in two broad sizes, nominally 500 strong (quingenaria) or 1,000 strong (milliaria). Their organisation as we understand it is summarised below, but we should assume that there was considerable variation even within unit types.

Size and Composition
Cohors peditia (Infantry)
480 to 600 men (all infantry)
6 centuriae of 80 to 100 men

Cohors peditia milliairia (Infantry)
800 to 1000 men (all infantry)
10 centuriae of 80 to 100 men

Cohors equitata (mixed unit)
600 to 720 men
120 cavalry (4 turmae) and 480 to 600 infantry (6 centuriae)
An adaptable formation suited to a range of tasks. The cavalrymen did not enjoy the same status as their counterparts in the alae

Cohors equitata milliaria (mixed unit)
1040 men
800 infantry (10 centuriae of 80) men and 240 cavalry (8 turmae of 30 men)

Ala quingenaria (Cavalry regiment)
480 men (all cavalry)
16 turmae of 30 cavalrymen

Ala milliaria (Cavalry regiment)
720 to 864 men (all cavalry)
24 turmae of 30 to 36 cavalrymen

Alongside these different types of unit, another formation emerges in the late first century, the numerus (pl. numeri). We still know little about these entities. The very vagueness of their title (numerus simply means unit) compounds our ignorance. They vary considerably in size – some as small as 200 men, others perhaps with ten times that number. They appear to be drawn overwhelmingly from non-citizens on the edge of empire or sometimes even from beyond the frontiers.

Having looked briefly at the organisation of the Roman army in the first and second centuries AD, we are now ready to look at how its soldiers would have appeared. We will start by looking at their equipment, and then go on to view the monuments they erected to commemorate their fallen comrades.

Mons Graupius: a decisive battle?

Outnumbered, but storming uphill through the ranks of their enemies, the soldiers of Rome destroyed Caledonian resistance in a final climactic battle. And remarkably, the battle was won almost without the loss of Roman blood, for the men delivering this lethal assault were not Rome’s famous legionaries, her citizens in arms, but auxiliaries brought to Britain from the tribes of the Rhine.

This is the climax of Tacitus’ biography of Gnaeus Iulius Agricola, governor of Britain. It is an account of combat 40 years before the building of Hadrian’s Wall at a place – still not definitively identified – known to the Romans as Mons Graupius (in the Grampian Hills) in what is now Scotland (Tacitus Agricola 29-38). In this account, Agricola emerges as an exemplary governor, a successful and innovative general (as here) and outstanding statesman. So compellingly is the man portrayed that it inspired future generations, and indeed as late as 1894, a fine statue of the great man was erected at Bath (see image above).

Praising Agricola’s brilliance, Tacitus relates how Rome’s famous citizen soldiers, the legionaries, were held in reserve as non-citizen auxiliaries were launched instead upon Rome’s Caledonian adversaries. He places particular emphasis on the Batavian and Tungrian auxiliaries, men recruited from two tribes of the Lower Rhine. The rationale, Tacitus assures us, was that by keeping his legionaries back, Agricola could hope to win without the loss of ‘Roman blood’, while if he suffered a reverse, he still had the legions in reserve. After an exchange of missiles, it was the Batavians and Tungrians, who, stabbing with their swords and striking with their shields, destroyed the enemy line, while auxiliary cavalry countered the enemy’s flanking attack and pursued them from the field.

How should we read this account of great generalship? It is clear that we should not take it at face value. Tacitus has an agenda in boasting of the achievements of Agricola; the man was his father-in-law after all! Some scholars have questioned whether the battle was anywhere near as decisive or as significant as Tacitus claims. We cannot identify the site for certain and debate continues as to precisely in which year the confrontation took place, AD 83 or AD 84. But the account is of more general interest – for it shows that a Roman readership could accept the idea that non-citizens could fight and win battles for Rome without the aid of citizen soldiers. And indeed, we will see that they often did.

Entry to the legions was not open to non-citizens, but they could enrol in the auxilia. By the time of the Roman conquest of Britain, it had become commonplace to award Roman citizenship to non-citizens who had served 25 years as auxiliary soldiers. This arrangement became a major force for social change. The children of some auxiliaries may have chosen to serve in the legions, but other avenues were also open to those with citizenship. Interestingly too, the auxiliary regiments themselves ended up with growing numbers of citizens enrolling in their ranks. In contrast to Tacitus’ claims, soldiers with ‘Roman blood’ were also to be found amongst the auxiliaries.

Historical accounts of battles are often vivid and exciting, but they are also notoriously problematic. Eye witness testimonies conflict, no one individual has total oversight, and the historian recording them may not even have been present. In a case such as this one, other agendas are also at play and some of these are drawn out in the preamble to the battle, a speech ‘reportedly’ from the lips of the Caledonian leader, Calgacus. His comments offer a wide ranging critique of his Roman adversaries, and lament the fact that they have even brought British allies with them to fight the Caledonians.

© Newcastle University

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