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Roman legions: the backbone of the Roman military

Feared across the Roman world and beyond, the Roman legions represented a pinnacle of military might. Why were they so successful? Could anyone join, and what training was required? Here’s all you need to know about the Roman legions…

A Roman soldier stands on guard during a re-enaction at Birdoswald Roman fort in Carlisle
Published: February 3, 2022 at 2:59 pm

What was a Roman legion?

If the Roman Army was the most powerful war machine in the ancient world, then the gears of that machine were the legions. In popular depictions they are envisaged as expertly trained soldiers, uncompromisingly disciplined, forged in the fires of myth as much as battle, and this iteration of the legion would come to embody Rome’s military prowess wherever it went. Yet this represents only a portion of the legion’s long history.

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Nor was that history always glorious. Despite their formidable reputation, the legions were not invincible, suffering famous defeats at Arausio, Carrhae and Cannae to name a few; at Teutoburg Forest in AD 9, three legions were annihilated in their entirety. But the fact that those defeats were so out of the ordinary has only served to enhance the legions’ reputation.

When was the first Roman legion raised?

The first reliable records of the Roman army come from the writings of the Greek historian Polybius, who describes the legions of the mid-second century BC.

At this time there were four Roman legions, but they were not the professional army we are most familiar with today – they were citizen militias raised by Rome’s consuls and, in their early history, disbanded after a single campaigning year.

The men who fought in these legions were drafted by ballot from all men of military age (17 to 46) who owned property more valuable than 3,500 sesterces. The stipulation about property ownership was important, as each man was expected to provide his own arms and armour.

The drafted men were assigned one of five roles within a legion, depending on their age or wealth. The wealthiest citizens served as equites (cavalry), while the poorest and those deemed too young to fight as infantry took the role of velites (skirmishers). The remainder were formed into three classes of infantry, each with slight differences in equipment – the hastati (the younger men), the principes (those in the prime of life) and the triarii (the veterans).

The hastati, principes, triarii would each form a battle line, split into maniples – the basic tactical unit, comprised of 120 men (or 60 men in the case of the triarii) and arranged in a chequerboard formation. They would be supported by two wings (alae) – units of allied troops from elsewhere in Italy, slightly larger than a legion and with a higher proportion of cavalry.

How did the Roman legions change with the Marian Reforms?

By the late second century BC, Rome had subjugated Macedonia and obliterated Carthage in the Punic Wars, but the seemingly ascendant Roman army was at risk of faltering – the militia system, which had proven so effective in the early days of the Roman republic, was not cut out for providing long-term garrisons in conquered provinces far from the Eternal City.

Enter Roman statesman Gaius Marius. In 107 BC, at which time he was consul, he approved sweeping changes to the Roman military.

It is unclear if Marius’s reforms were simply formalising changes that had been used intermittently on prior occasions, but in any event they were transformative. He abolished the property requirement, allowing the capite censi ­– those ‘counted by the head’ in the ancient Roman census, a non-landing owning class – to join the ranks of the Roman army for the first time. Because these men were poor, he placed the responsibility (and cost) of supplying weapons and armour on the state.

The ranks swelled: for the landless lower classes, military service guaranteed a pension, regular meals and good medical care. The result was that Rome, for the first time in its already long history, could now call upon a professional standing army.

But it would also contribute to the downfall of the republic and the birth of the Roman empire, because whereas the landed levies of the pre-Marian legions had a vested interest in the Roman state, the capite censi had no such motivation. Their loyalty tended to be to the generals who armed them, fed them and led them to glory, exemplified perfectly when Julius Caesar crossed the Rubicon.

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The assassination of Julius Caesar

How was the Roman Legion structured?

The Marian Reforms removed the five classes of soldier and abolished the maniple. All recruits were now heavy infantry, equally trained and equipped.

The basic unit of the new legion was the cohort, totalling 480 men. A cohort was comprised of six centuria, or centuries, each led by a centurion; confusingly for modern sensibilities, a century totalled 80 men rather than 100. The centuria were further divided into contubernia, the smallest subdivision of the legion. This was a group of just eight fighting men who would do everything together, from sleeping in the same tent and sharing the same cooking pot to fighting side by side in battle.

A single Roman legion was made up of nine such cohorts, plus one ‘double strength’ cohort of five double-strength centuria, comprising 800 of the most experienced soldiers. The centurion of the first century of the first cohort outranked all others in the entire legion, and he was known as the primus pilus – the ‘first spear’.

Why were centurions so important to the legion?

 If the legions were the backbone of Roman military power, then the centurions were the backbone of the legions. Recognisable by their distinctive plumed helmets, they were responsible for both commanding their century, and administering its training and discipline. It was a prestigious role, but a dangerous one – in battle, the centurion was expected to lead from the front.

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How many men were there in a Roman legion?

It depends when in Rome’s history we are referring to. In the days of the early republic, each legion was 4,200 men, expanding to 5,000 men in times of particular peril.

After the Marian Reforms and during the Principate period of the Roman empire, each legion numbered approximately 5,200 men, sometimes rising to 6,000. This diminished to as few as 1,000 men during the empire’s twilight years.

These are all paper strengths, however. The legions were rarely at full manpower owing to illness, retirement and losses at war, though the flexibility of the cohort structure meant they were still able to function effectively.

What were Roman auxiliaries?

The auxiliaries, or auxilia, were introduced by the emperor Augustus and hailed from all over the Roman empire. They fought alongside the legions, with their precise roles – archer, skirmisher, slinger or cavalry – depending on where they were raised.

They were formed into contubernia, centuria and cohorts, each of which had its own Roman commander. Unlike the legionaries, the auxilia were not Roman citizens; citizenship was their reward for completing 25 years of service.

What ranks were there in the Roman legion?

Though the best-known roles are the legionary and the centurion, there were myriad other ranks in the Roman legion. Here are a few of them:

Legatus legionis | The legatus legionis (legate) was the overall legionary commamder, and was usually a former tribune (see below). Sometimes, the legate was also the provincial governor of the regions in which their legion was based.

Tribunus | There were six military tribunes attached to each legion in post-Marian era: five narrow-stripe tribunes (tribunus angusticlavius) and one broad-stripe tribune (tribunus laticlavius), the latter serving as the legate’s second in command.

Praefectus castoreum |The camp prefect and third most senior officer in the legion, whose duties included procuring equipment and directing construction of the legion’s encampment.

Aquilifer | The man who carried the silver eagle (later gold eagle), the legion’s most important standard, was the aquilifer; consequently, he was also the most important of the legion’s standard bearers (signiferi). The loss of an eagle was not something to be tolerated; the legion was expected to fight until its own destruction rather than surrender it.

Imaginifer | Another of the signiferi, introduced after the formation of Roman empire, this man bore an image of the emperor to remind the legionaries of their oaths and where their loyalties lay. The post was first established during the reign of Augustus.

Tessarius | The tessarius was the guard commander. It was his job to circulate the tessara (a small tablet) holding the daily watchword.

What were a Roman legionary’s weapons and armour?

 Three weapons are most famously associated with Roman legionaries in the post-Marian period – the pilum, gladius and pugio. The pilum was a javelin that was hurled at the enemy as the legion advanced, and was fashioned in a such a way that it would bend on impact, hopefully rendering an opponent’s shield useless. Combat then largely relied on the gladius hispaniensis – though it was double-edged, the 60cm blade was primarily used as a stabbing weapon, rather than for sweeping cuts, as this style of fighting both conserved energy and was more likely to land a fatal blow. As a sidearm, some legionaries bore a short dagger, or pugio.

Roman soldiers wore many types of armour, or lorica. The most recognisable in popular depictions is segmented plate (lorica segmentata) but chainmail and scale armour were in use for many years. Soldiers wore a leather belt (balteus) over their armour, which could hold their weapons. The best-known helmet is probably the galea, which had cheek guards for added protection as well as a trim for the neck and forehead. A horsehair plume was sometimes added to give the legionary a taller, more imposing appearance.

Shields of varying shapes were used through the history of the legions, but the most familiar was the rectangular and slightly curved scutum – a shield large enough it could cover the legionary’s body entirely. With a heavy iron boss at its centre, it became an effective punching weapon in its own right.

What was the testudo?

One famous battle formation that made use of the scutum was the testudo, or tortoise. The legionaries would lock their shields both in front and above their heads – forming a shell that allowed the whole unit to advance safe from projectiles from above.

What requirements were there for becoming a legionary?

All hopeful legionaries had to be Roman citizens, in good physical shape and a minimum of six Roman feet (5ft 9 inches). Though not a requirement, recruits from the countryside were preferred to those from urban areas, as it was thought that they were more accustomed to physical labour and would bear the hardships of war easier.

Among the more ephemeral considerations, the Roman writer Vegetius notes that “lively eyes” might be a good indicator of a keen mind. Literate recruits were particularly welcome, as the running of each legion required a hefty amount of administration. Those with specialist skills or trades could hope to avoid latrine digging duty and other manual labour to become immunes – engineers, carpenters, musicians and so on. Letters of recommendation from other influential Romans didn’t hurt either.

How were legionaries trained?

Legionary training lasted for four months, beginning with marching, with recruits expected to cover 18 miles in five hours while carrying loads of around 45lbs. This was critical to teaching soldiers how to maintain their ranks over difficult ground, when advancing while be attacked with projectiles, or while changing formation.

Combat training began with sparring against a wooden post, using a wooden sword and shield that were twice as heavy as their real counterparts – building strength and endurance that would make their actual weapons easier to handle. Later the recruits would fight each other one on one, or in mock battles; sometimes they would spar with gladiators. Javelin throwing, slinging and swimming were also core elements of the training regimen.

How was discipline maintained – and was decimation real?

Discipline was rigorously enforced, usually at the capricious hands of the centurions, whose vitis (vine staff) doubled as both a symbol of office and a disciplinary rod. Even minor infringements were likely to incur a beating, while more serious infringements were subject to the death penalty. These included sleeping while on sentry duty – easily achieved if you used your pilum to prop up your scutum – the punishment for which was fustuarium. The sleepy soldier would be cudgelled to death by the men whose lives were endangered.

The most famous punishment, reserved for larger-scale acts of desertion, cowardice or insubordination, was decimation. Though the word is more broadly used today as a synonym for ‘destroyed’, in the Roman era decimation had a more specific meaning: ‘removal of a tenth’.

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A group of legionaries subjected to decimation would be made to draw lots; those drawing the proverbial short straws would be put to death, often at the hands of those who survived the lottery. The remaining nine out of ten men would be put on barley rations (instead of wheat) and made to sleep outside the camp.

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