The Imperial Legion: Most complete unit

The Roman Legion was one of the most vaunted military units in history. However, from the adoption of the Legionary structure, till the fall of the Western Empire. However, the legion underwent structural and tactical changes throughout time.

For the purpose of this article, we are considering the evolution of the legion from the Polybian to Marian to the Imperial legions only. The Polybian Legion was adopted when Rome started to expand into hilly terrains of the Italian peninsula. The Marian legion was formed towards the last century of the Republic, when the need to arm the expanding Roman military machine in an efficient manner. The Imperial legion was constituted by Augustus, when Rome approached its zenith and needed a permanent force to govern its provinces.

In my opinion, a single full strength Imperial legion was the most complete war formation until the Napoleonic Army’s corps. A single legion, could besiege towns or fight large forces itself. Of course, there were some drawbacks, but no other civilization in between could claim to have a single military formation that was as effective as the legion for all-round purposes.

The article, which might be long will focus broadly on the changes underwent and what made the Imperial legion a force to be reckoned with.


For information about the Polybian Legion and why it was superior to the Phalanx, please read my articles that are linked on the page here and here.

Broadly, the characteristics of each type of Legion are listed below.

The Polybian Legion and its battle formation.

Polybian Legion: Strength of 4200 men. 1200 skirmishers, 3000 heavy infantry grouped into 30 centuries of 100. The infantry were grouped by age and equipment. There were 1200 of the relatively lightly-heavy armoured, 1200 medium-heavy infantry and 600 heavily armed men. A consular army of 2 legions were accompanied by similar number of allied infantry and 2400 cavalry. All soldiers bought their own equipment.


Marian Legion: Strength of around 5000 men. Men are divided into 10 units, called Cohorts. All soldiers are similarly armed and carry food, cooking equipment and other necessary items to reduce the baggage train. Equipment was provided by the state now. Accompanied by similar number of allied infantry.


Imperial Legion: Strength of 5120 men. 10 cohorts, with ‘First Cohort’ consisting of 800 instead of 480 men. This was a professional army with a fixed 25 year serving term. Soldiers could rise up the ranks. Armament was similar to the Marian legion. Legion had 120 light cavalry and a mix of 1:1 or 1:1.5 in terms of various auxiliaries and cavalry.


What made the imperial legion more powerful than its predecessor structures?

First of all, let’s see why the Marian Legion was superior to the Polybian legion. The Marian legion was more standard, with all soldiers similarly armed. Imbalance was no longer any issue. A standard Marian and Imperial legion could easily be replenished or double sized if the situation demanded.

Secondly, was tactical. Instead of a maniple of 100 men as the main unit, a cohort of 480 men formed the tactical unit. This was large enough to respond to various smaller-scale situations and could function like a mini-legion. Situations such as besieging a small fort, or allowing the legion to spread out across a larger area in search of the enemy, or maintain combat readiness across various terrain.

Thirdly, logistics. The soldiers from the Marian legion were known as Marius’s mules. With the help of a Y-stick they could carry food, armour and other essential ingredients without much effort. This reduced the length of supply trains of the legions and improved their overall marching speed. It was also possible to rapidly pursue any foe for a few days without losing supplies.

The Marian legion created a single type of heavy infantry to be used.

While not official, most of the Marian legions were Roman citizens who were to be discharged with a plot of land. During the expansionary phases of the empire, it was a brilliant solution to romanising a province and paying off soldiers who completed their tenure. However as the empire reached its peak, it was more difficult to buy or acquire land. The Imperial Legion solved some other short comings of the Marian legion.

Firstly, the Imperial legion represented a professional standing army. Sure with the number of civil wars in the last century of the Republic meant soldiers served for years at a stretch, the Imperial Roman Legion had men serving 20 years actively and 5 years as a reserve (though it’s been known that men served longer in the legions).

Secondly, legions were now permanent garrisons. This added security across the empire, and provided infrastructure labour for the provinces such as construction projects, road development, etc. It was easier to respond to threats such as the Germanic tribes or incursions across the Danube River.

Third, the discharge. In lieu of a plot of land, Soldiers were given a discharge bonus, equivalent to 13 years gross salary of a standard legionary. Assuming a soldier served the full 25 years, they were paid 38 years of salary, with any additional war prizes earned during a campaign. This made the legions an attractive source of employment.

Fourth, Auxiliaries. With the Republican Socii being annexed and converted into Italian citizens, support was needed for the heavy infantry. From the auxiliaries, cohorts of light infantry, archers, skirmishers and cavalry (and camel riders for the Syrian province) were organized. The force accompanying each legion varied in composition and strength, but overall gave a legion a core of heavy infantry and supporting units to effectively combat any situation.


The Imperial legion also had siege capabilities. Every century had a scorpion bolt thrower, along with an additional legionary complement of 10 catapults and 20 bolt throwers. With its auxiliaries, it was an easy matter of besieging any town and working in tandem with other legions to besiege larger cities. The siege of Jerusalem by Vespasian had over 350 pieces of artillery to pound the rebels within the city.

Emperor Augustus was the one who formed the Imperial legionary structure

In overall strength, there were 5120 men in a legion, with 120 light cavalry and auxiliaries. The ratio of Auxiliaries to legionaries varied from 1:1 to 1.5:1, thus a legion may have had up to 12,000 men including heavy infantry, cavalry, archers, siege equipment.

No army at the time had any unit as complete as a legion. Sure overall armies could fight, but a single unit could easily branch out. Thus on campaign, the Romans did not have to spend much time trying to effectively divide their forces if needed. It was already done and ready.

The structure also allowed to fight in forests (with limited effectiveness of any standard army has against guerrilla warfare), plains, confined areas and cities. This meant the Romans were more used to butchering their opposition than getting butchered themselves. Of course there were instances where they were annihilated, such as the battle of Tuetoburg forest, but that was a very well planned ambush that was carefully executed in pre-prepared forest trail.

An archer was one of the auxiliaries of Rome

For these reasons of evolution, the Legion was a single military unit that was the most complete sub-unit of an army in warfare until the arrival of the corps in the Napoleonic Army.


If you disagree, I would love to hear your points regarding the same!


Roman Imperium.

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