The Imperial Roman army

A roman legionary from the 1st or 2nd century (Museum Het Valkhof, Nijmegen).

A Roman legionary from the 1st or 2nd century (Museum Het Valkhof, Nijmegen).

Although the Roman army had already been transformed from a conscript army into a professional fighting force during the Late Republic, we must credit Augustus with the creation of a professional standing army. In the year 6, he established the aerarium militare, the military treasury from which the soldiers were to be paid. Legions became permanent units with permanent names and numbers (e.g. Legio III Augusta, Legio XX Valeria Victrix), and this led to greater unit pride within the ranks. But who actually served in the legions and, more generally, in the Imperial Roman army? How did the makeup of the army evolve over the centuries and how did the nature of warfare change?

Makeup of the Roman army

In the early Imperial period, Roman armies were made up of legions and cohorts of auxiliary troops Auxiliary units were called alae if they were cavalry and there were also mixed cohorts of infantry and cavalry, the so-called cohortes equitatae. Only Roman citizens were allowed to serve in the legions, so these comprised only soldiers from Italy and from the communities in the provinces which had been granted Roman citizenship, such as the Germanic Batavi. Citizenship could be granted by the emperor, usually to whole communities who had served Rome well. It could also be earned by serving in the auxiliary infantry or cavalry for 20 or more years. Children of former auxiliaries would then also receive citizenship and become eligible for service in the legions.

Bust of Caracalla (Capitoline Museums, Rome)

Bust of Caracalla (Capitoline Museums, Rome)

In this way the provincial element in the legions was increased. During Augustus’ reign (27 BCE-14 CE), the majority of the soldiers still came from Italy, but 100 years later the situation was very different. In Trajanus’ days (98-117) only one in four soldiers would be from Italy, with Italians becoming more and more unwilling to serve. The rest of the legionaries came from the provinces. The most drastic change came during the reign of the emperor Caracalla (211-217 AD), who in the year 212 granted citizenship to the vast majority of the inhabitants of the provinces (the so-called Constitutio Antoniniana). Since practically everybody was now allowed to serve in the legions, the old distinction between legionaries and auxiliaries disappeared.

Crisis of the Third Century

The so-called Crisis of the Third Century started soon after the death of Septimius Severus in 211. It was a period of weak emperors, who were constantly toppled, raids by Germanic peoples deep into the Empire and the emergence of a new superpower in the East, the Sassanid Persians, who had overthrown the Arsacid Parthian dynasty by 224. Emperors died on the battlefield, like Decius against the Goths. Some were even captured by the enemy, like Valerianus by the Persian King Shapur. This crisis brought about the need for reform of the army, which was already started under Valerianus and his son Gallienus (253-268) and was not completed until the reign of Constantine the Great (306-337) many decades later.

There were two main reasons for the reforms. One was the growing awareness that the old army made up of legions of some 5.000 men was not fit to deal with raids. It was an army that excelled at fighting large pitched battles, but these were rare in these days, especially in the West, where raids were far more frequent. A distinction was thus made between border troops (limitanei) and the mobile field army. The border troops were to stop the small-scale invasions in their tracks and they seem to have generally been successful. The limitanei were certainly not poorly equipped peasants or militia, but soldiers in the regular army who received a formal training, even though they were obviously not as well drilled as, say, Caesar’s legions. If the raiding armies managed to break through, the mobile field army could try to intercept them. “Mobile” means the troops were not confined to a fixed location. It does not mean the soldiers were all mounted. Although the percentage of cavalry in Roman armies was increased from the middle of the third century, cavalry continued to play the traditional role of supporting the infantry, which lasted until well into the fifth century.

The nature of warfare in the West and the East

Bronze face mask of an auxiliary horseman, 2nd century (Rijksmuseum van Oudheden, Leiden).

Bronze face mask of an auxiliary horseman, 2nd century (Rijksmuseum van Oudheden, Leiden).

The nature of warfare changed. The Roman army adopted tactics which can be qualified as “guerrilla methods”: low-level operations involving smaller units (a legion in those days would have comprised no more than 1.000 men), setting ambushes and striking when the enemy was trying to get back across the borders. These tactics generally seem to have been successful. The situation changed drastically when Germanic peoples began to cross the borders to settle on Roman soil in late fourth century. These were not raids anymore, but migrations aimed at finding new homelands. A stable Roman Empire would probably have been able to stop these “barbarian invasions” by fighting them off. The Empire – and before that, the Roman Republic – had been able to do just that for centuries. But constant civil strife had made the Empire very weak internally. Emperors now had to bribe peoples like the Goths and donate them land to settle on in return for soldiers who would fight in their armies. This can been seen as the period of “barbarisation” of the Roman army. These people came from outside the Roman Empire and were not (at least not originally) inhabitants of the provinces.

The situation would have been a little different in Asia Minor, Syria and the Levant, where the Empire was locked in a centuries long stalemate with the Persians and where mass migrations were absent. Although raids were frequent here as well, Persian armies regularly invaded and occupied Roman territory. Contrary to the Parthians with their lack of knowledge of siege warfare, the Sassanids were pretty good at it, overrunning much of Syria and even Asia Minor in the 260s until the short-lived separatist Kingdom of Palmyra drove them back. This was all the work of a Palmyran noble called Septimius Odaenathus, who drove the Persian King Shapur back to his capital Ctesiphon and forced him to stay on the defensive until his death in 272. For his actions, Odaenathus was named dux and commander of all of the East (corrector totius orientis), but he was murdered in 267. He was succeeded by his son Vaballathus, but it was his widow Zenobia who gave the Romans a headache by effectively ruling the eastern part of the Empire, ignoring the legitimate emperors in the West and naming herself Augusta. She was ultimately defeated by the capable Lucius Domitius Aurelianus, who had become emperor in 270. It is with Aurelianus that we see a resurgent Roman Empire and it is with him that the Crisis of the Third Century finally came to an end.

In the East, the question was mainly about finding the right weapons to counter the enemy, who relied heavily on ranged weapons. From the days of Crassus’ disastrous expedition against the Parthians in 53 BCE, the Romans had realised that every army of theirs needed proper support by archers, slingers and cavalry, so these troops were already more present in the East than elsewhere. Roman expeditions against the Parthians had generally been successful. The Parthian capital Ctesiphon had been captured by Trajanus in 116, by Avidius Cassius in 165 and by Septimius Severus in 198, although the Romans seldom managed to fully annex the territories they conquered. The point, however, is that while Roman armies managed to take the Parthian capital on three occasions, no Parthian army ever came close to Rome. What did change towards the end of the second century was the introduction of heavy cavalry in the Roman army in the form of cataphracts and clibanarii, which were also used by both Parthians and Persians.

Sassanid cataphract (photo: John Tremelling).

Sassanid cataphract (photo: John Tremelling).

Keep the throne secure

The second reason for the reforms and the distinction between border troops and a mobile field army was the need for the emperor to have loyal troops near Rome. It is certainly no coincidence that already under Septimius Severus more legions were stationed in Italy than before. The emperor needed an army at his immediate disposal, because the previous centuries had shown that his throne was never secure and he needed to protect it against potential rivals.

With a change in the nature of warfare came a change in equipment. The famous Roman short sword – the gladius – was ultimately exchanged for the longer spatha. The equally famous but somewhat clumsy lorica segmentata disappeared as body armour and chainmail became dominant again, although it had never been fully abandoned anyway. The same is true for the change from rectangular shields (scutum) to lighter oval shields. We must realise that oval shields never completely disappeared from the legions, and were already omnipresent in the auxiliary forces. The armies of the early Principate generally threw one volley of missiles and then charged with their swords in their hands. In close combat, the heavy legionary shield offered great protection and could also be used offensively. However, during the third century the Romans changed (or rather: had to change) their battlefield tactics as I described above.

Battles now usually involved long exchanges of missiles. The heavy throwing spear (pilum) was phased out and legionaries began carrying multiple shorter and lighter missiles. The plumbata or mattiobarbuli (leaded darts) are probably the best known types of missiles. The shields of this period had special slots for holding the missiles. These shields were generally oval or round, a lot lighter than the heavy scutum (which itself was considerably lighter than the oval and convex shield used during the Republic) and flat. So in sum, changing tactics necessitated different equipment.

Detail from Constantine's Arch in Rome. Note the round shields of the soldiers.

Detail from Constantine’s Arch in Rome. Note the round shields of the soldiers.

Even though low-level operations became more common, the Roman army was certainly not in decline and was still able to defeat much larger enemy forces. For instance Julianus’ 13.000 men decisively defeated an Alemanni force of 35.000 in the Battle of Strasbourg (357), even though the Romans lost the initial cavalry battle. The Roman army evolved from a citizen militia to a conscript army, and subsequently to a semi-permanent professional army and a professional standing army. The standing army was also frequently reorganised, its makeup and equipment changed over the centuries, but it continued to be the backbone of the Roman Empire and the ultimate base of power for any emperor. The Roman army, like Rome itself, was not created in a day. And it certainly was not destroyed in a day.


  • Bernard van Daele, Het Romeinse leger;
  • Adrian Goldsworthy, The Complete Roman Army;
  • Adrian Goldsworthy, In the Name of Rome.

This essay is a revised version of a post I wrote back in 2008 on this forum.


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