The Roman legions were the fighting force which allowed Rome’s territories to expand across three continents. However, it took several centuries for the Roman army to form into a fighting force which would take Rome's territories far beyond the Italian peninsula.

The legions that many people think of were not actually established until 107 BCE under the Marian reforms. Before this date, the legion was used to refer to the entirety of the Roman army. There are three distinct periods in Roman military history: an early era in which the phalanx formation was used. The maniple formation used between 315 BCE and 107 BCE and the cohorts used from 107 BCE onwards.

Early Era

During the Roman Kingdom and early Roman Republic (753 BCE to 315 BCE), Rome utilized the Greek-style phalanx formation. This style of fighting was prevalent throughout Italy at the time of Rome's conception and was used for several centuries.

The majority of the army was infantry based who were equipped with a spear and shield. The soldiers were tightly packed, and each man would protect the soldier to his left with his shield. Each phalanx unit would be sixteen men deep. After the initial impact, the soldiers behind the first line would begin to push, forcing the first line to fight and preventing any gaps in the formation from appearing.

According to Polybius, the phalanx was the perfect formation when fighting on a battlefield which was at least 3,500 meters in width with no obstacles which could prevent flanking movements. Despite being a formidable formation face on, if flanked or reared it was incredibly fragile.


In 315 BCE, during the Samnite Wars, the phalanx was replaced by the maniple. This was a much more flexible system that was not hindered by rough terrain and was vital in Rome's victory over Samnium.

There were several changes to weaponry and armor: the hoplon shield was substituted for a scutum shield. Which better allowed for a soldier to defend himself rather than relying on the man to his right. The spear (hasta) was substituted for the sword (gladius) and javelin (pila).

Four new units were used: the Velites, Hastati, Principes and the Triarii.

  • Velites - were very lightly armored infantry armed with javelins (pila) and a short sword. They would be positioned at the front of the Roman battle line. They would launch their pila at the enemy before falling behind the other lines.
  • Hastati - were medium armoured infantry armed with a sword (gladius). They would serve as the front line of the army and would be first into the hand to hand combat. These were among the youngest soldiers and were eager to prove themselves in battle.
  • Principes - were heavy infantry armed with a sword (gladius) and better armor than the Hastati. They were the second line of infantry behind the Hastati. These men were in their late twenties and had considerable experience on the battlefield.
  • Triarii - were heavy infantry armed with a sword (gladius) and better armor than the Principes. They were the final line of infantry and would only join the battle if they were required. Usually, the two front lines of the army would be able to finish the fight before the Triarii were required. These were the most experienced men in the Roman army.

There were fifteen maniples in each line all consisting of eighty men commanded by a centurion. On arriving at the battlefield, the Roman army would set up in a chequered formation, resembling a chess board. The Velites at the front, the Hastati as the front line of infantry, the Prinipes as the second line of infantry and the Triarii at the rear of the formation.

Once the battle began the Velites who were deployed on the front line would launch their pila as soon as the enemy came within range. After throwing all of their projectiles, they would fall back between the gaps in the chequered formation.

The Hastati would then launch their pila before engaging in melee combat. If the battle was not going in Rome's favor, the Hastati would fall back behind the Princeps. The same would apply to the Princeps if they were under too much pressure they would fall back to regroup and allow the most experienced units, the Triarii, to be deployed.

The flanks of the army were protected by three hundred cavalrymen. The cavalry performed various functions: they were to strike the enemy's flank and rears in swift assaults before retreating to strike again. Secondly, they were used in skirmishes and to chase down any fleeing enemy. Thirdly, they were used for reconnaissance and intelligence gathering.


In 107 BCE, Gaius Marius was elected consul and introduced the Marian reforms, broad sweeping changes to the military which transitioned it into the legions which helped dominate and maintain the Rome's domains.

The Roman army was organized into legions of 4,800 soldiers. Each legion was split into ten cohorts of 480 soldiers. This added flexibility as all ten cohorts could be brought together to fight as one entity or be divided into smaller forces each capable of operating on its own.

On the other hand, it would take several hours for the cohorts to march from camp to their formation on the battlefield, due to the size and complexity of the legion. To reduce the time it took, the cohorts would march in a specific order. The cohort located on the far right of the battle line would march first followed by the unit located to its left and so on. Once the legion arrived at the battlefield, the first cohort would stop and march to the right until the entire Roman line was in position. The Roman formation would unfold like a snake. For enemies who had never faced Rome would undoubtedly be intimidated by the sheer organized nature of the legion.

In the first century CE, Roman skirmishing units were replaced by auxiliary units. For example, Cretan archers and Numidian javelin throwers. Additionally, when Augustus established the auxiliaries in around 30 BCE, the Roman cavalry was replaced with auxiliary cavalry from the provinces.

The cohorts were a flexible system which could be deployed in numerous ways. However, the preferred deployment was to have a center of two legions which were flanked with auxiliary infantry. This was then supported by cavalry on the flanks. This was used at the Battle of Watling Street, where Rome won the defining battle against Boudicca despite being heavily outnumbered.

The Roman legion could change to accommodate for factors such as terrain, enemy, and weather. Vegetius wrote of various formations used by the Roman army.

  • A single long line which could be used on open battlefields where the Roman army outnumbered the enemy. The Roman line would envelop the enemy and completely surround them.
  • A line which would form an obtuse angle, similar to an 'L'. One wing of the legion would engage the enemy, and then the other wing would attack the exposed flank and rear of the enemy.
  • A line where both flanks of the enemy were more densely packed with the center being weaker. This would allow the enemy to break through the soft center and the wings could envelop the enemy.

It would be the general's responsibility to choose the most efficient formation based on the relevant factors. He was ultimately responsible for the success or failure of the legion.

The battle would start with the front lines launching their pila into the enemy before moving back into their compact battle formation. The front lines would then charge the enemy; this would result in a rush of adrenaline, and the impact would hopefully break the enemy providing an easy victory. Only the soldiers at the front of the formation would fight hand to hand; this would leave a majority of soldiers out of combat and rested. After short periods the commander of the unit would issue the order for the men at the front to step back and for the men behind to take their place. By constantly cycling the soldiers it would keep them fresh and avoid fatigue. This granted the Roman legions an advantage the longer a battle continued.

Naming Roman Legions

There were several ways in which Roman legions were named and numbered. Which causes a headache for historians trying to map them...

  • Location - based on where the legion was located. For example, 'I Germanica' (Germany)
  • Enemy - based on who the legion was fighting. For example, 'I Parthica' (Parthians)
  • Emperor - based on the emperor who was ruling when it was established. For example, 'II Augusta' (Augustus)

Certain numbers and titles were avoided for superstitious reasons. For example, following the humiliating defeat at Teutoburg Forest the numbers: XVII, XVIII, and XIX were no longer used when numbering legions. Additionally, if a legion was triumphant on a campaign, they would be granted certain titles. For example, victrix meaning victorious.

List of Roman Legions

Legion Title Located Date Founded Date Disbanded Emblem
I Adiutrix Hungary 68 CE, Nero 444 CE Capricorn
I Germanica Germany 48 BCE, Caesar 70 CE Bull
I Italica Bulgaria 66 CE, Nero After 400 CE Boar
I Minerva Germany 82 CE, Domitian After 300 CE Minerva
I Parthica Iraq 197 CE, Severus 400 CE Centaur
II Adiutrix Hungary 70 CE, Vespasian After 250 CE Capricorn
II Augusta Wales 10 BCE, Augustus After 300 CE Capricorn
II Italica Austria 165 CE, Aurelius After 400 CE She-wolf
II Parthica Italy 197 CE, Severus After 350 CE Centaur
II Traiana Egypt 105 CE, Trajan After 400 CE Hercules
III Augusta Algeria 43 BCE, Augustus After 350 CE Pegasus
III Cyrenaica Syria 36 BCE, Antony After 400 CE Unknown
III Gallica Jordan 49 BCE, Caesar After 300 CE Bull
III Italica Germany 165 BCE, Aurelius After 300 CE Stork
III Parthica Syria 197 BCE, Severus After 400 CE Bull
IV Flavia Felix Serbia 70 CE, Vespasian After 300 CE Lion
IV Macedonia Germany 48 BCE, Caesar After 70 CE Bull
IV Scythica Turkey 42 BCE, Antony After 400 CE Capricorn
V Alaudae Germany 52 BCE, Caesar After 90 CE Elephant
V Macedonia Romania 43 BCE, Augustus After 500 CE Eagle
VI Ferrata Israel 58 BCE, Caesar After 250 CE She-wolf
VI Hispana England After 200 CE After 250 CE Unknown
VI Victrix England 41 BCE, Augustus After 400 CE Bull
VII Claudia Serbia 58 BCE, Caesar After 400 CE Bull
VII Gemina Spain 68 CE, Galba After 400 CE Unknown
VIII Augusta France 59 BCE, Caesar After 350 CE Bull
IX Hispana England 41 BCE, Augustus After 125 CE Bull
X Fretensis Israel 40 BCE, Augustus After 400 CE Boar
X Gemina Austria 42 BCE, Lepidus After 400 CE Boar
XI Claudia Bulgaria 42 BCE, Augustus After 400 CE Neptune
XII Fulminata Turkey 43 BCE, Lepidus After 400 CE Thunderbolt
VII Gemina Spain 68 CE, Galba After 400 CE Unknown
VIII Augusta France 59 BCE, Caesar After 350 CE Bull
IX Hispana England 41 BCE, Augustus After 125 CE Bull
X Fretensis Israel 40 BCE, Augustus After 400 CE Boar
X Gemina Austria 42 BCE, Lepidus After 400 CE Boar
XI Claudia Bulgaria 42 BCE, Augustus After 400 CE Neptune
XII Fulminata Turkey 43 BCE, Lepidus After 400 CE Thunderbolt
XIII Gemina Romania 57 BCE, Caesar After 400 CE Lion
XIV Gemina Austria 41 BCE, Augustus After 400 CE Capricorn
XV Apollinaris Turkey 41 BCE, Augustus After 400 CE Apollo
XV Primigenia Germany 39 CE, Caligula After 70 CE Fortuna
XVI Flavia Firma Turkey 70 CE, Vespasian After 300 CE Lion
XVI Gallica Germany 41 BCE, Augustus After 70 CE Lion
XX Valeria Vict. England 31 BCE, Augustus After 250 CE Boar
XXI Rapax Switzerland 31 BCE, Augustus After 90 CE Capricorn
XXII Primigenia Germany 39 CE, Caligula After 300 CE Hercules
XXX Ulpia Victrix Germany 105 CE, Trajan After 400 CE Jupiter