The Roman Legion (from Latin legio "military levy, conscription," from legere — "to choose") is a term that can apply both as a transliteration of legio ("conscription" or "army") to the entire Roman army and also, more narrowly (and more commonly), to the heavy infantry that was the basic military unit of the ancient Roman army in the period of the late Roman Republic and the Roman Empire. In this latter meaning, it consisted of several cohorts of heavy infantry known as legionaries. It was almost always accompanied by one or more attached units of auxiliaries, who were not Roman citizens and provided cavalry, ranged troops and skirmishers to complement the legion's heavy infantry.
The size of a typical legion varied widely throughout the history of ancient Rome, with complements of 4,200 legionaries and 300 equites in the republican period of Rome, (the infantry were split into 30 maniples of 120 legionaries each), to 5,200 men plus auxiliaries in the imperial period (split into 10 cohorts, 9 of 480 men each, plus the first cohort holding 800 men).
As legions were not standing armies until the Marian reforms (c. 107 BC), and were instead created, used, and disbanded again, several hundred legions were named and numbered throughout Roman history. To date, about 50 have been identified. In the time of the Early Roman Empire, there were usually about 25-35 standing legions plus their Auxiliaries, with more raised as needed. See List of Roman legions for a catalogue of known late republic, early Empire and late Empire legions, with dates in existence, emblem and locations of deployment.
Much of Roman history of this era is founded on legends, but it is believed that during the reign of Servius Tullius, the census (the counting of the people) was introduced. With this, all Roman able-bodied, property-owning male citizens were divided into five classes for military service based on wealth, soldiers having to acquire their own weapons and equipment. These classes were further organized into units of 100 called centuries (who also had a centurion, a second in command and a standard bearer).
Joining the army was both a duty and a distinguishing mark of Roman citizenship: during the entire pre-Marian period the wealthiest land owners performed the most years of military service, since these would have had the most to lose should the state have fallen. The first class was armed in the fashion of the hoplite with spear, sword, helmet, breast plate and round shield (called clipeus in Latin, similar to the Greek aspis, also called hoplon); there were 82 centuries of these of which 2 were trumpeteer. The second and third class also acted as spearmen, but with reduced armour, and the larger oval or rectangular shield. The fourth class could afford no armour, perhaps a small shield, and was armed with spear and javelin. All three of the latter classes made up about 26 centuries. The fifth and final class was composed only of slingers. There were 32 of these of which 2 were engineers. The army officers as well as the cavalry were drawn from leading citizens who enrolled as equestrians (equites). (The equites were later put in smaller groups of 30 and were commanded by decurions (which strangely means commander of ten)), there were 18 centuries of equites.
Tactics were no different from those of the early Greeks and battles were joined on a plain. Spearmen would then deploy themselves in tightly packed rows. The members of each row closed to form a shield wall, their spears pointed forwards, and charged the enemy supported by their javelin throwers and slingers: the cavalry pursued the enemy, and sometimes dismounted to support the infantry in dire situations. It was a cumbersome military unit to manoeuvre, and easily defeated by mountain tribes such as Volsci or Samnites in rough terrains.
Early civilian authorities, called praetors, doubled as military leaders during the war season; lasting from spring to autumn. A formal war declaration included a religious ceremony that ended in the throwing of a ceremonial javelin into the enemy's territory to mark the start of hostilities.
At some point, possibly in the beginning of the Roman Republic after the kings were overthrown, the legio was subdivided into two separate legions, each one ascribed to one of the two consuls. In the first years of the Republic, when warfare was mostly concentrated on raiding, it is uncertain if the full manpower of the legions was summoned at any one time. Legions became more formally organized in the 4th century BC, as Roman warfare evolved to more frequent and planned operations, and the consular army was raised to two legions.
In the Republic, legions had an ephemeral existence. Except for Legio I to IV, which were the consular armies (two per consul), other units were levied by campaign. Rome's Italian allies were required to provide a legion to support each Roman Legion.
In the middle of the Republic, legions were composed of the following units:
Each of these three lines was subdivided into maniples, each consisting of two centuries of 60 men commanded by the senior of the two centurions. Centuries were normally 60 soldiers each at this time in the hastati and principes (no longer 100 men), with 120 strong maniples. There were generally 10 maniples of hastati, 10 maniples of principes and 10 of triarii which had two half strength centuries of 30 men, plus about 1200 velites and 300 cavalry which made up 10 units - 30 men strong. This gave the mid Republican legion a nominal strength of about 4500 men. Later on when the legions undertook the Marian reforms and was made up of 80 strong centuries each century had its standard and was made up of ten units called contubernia. In a contubernium, there would be eight soldiers who shared a tent, millstone, a mule and cooking pot (depending on duration of tour). Because maniples were their main tactical elements, the legions of the early Republic are sometimes referred to as Manipular legions.
Following the Marian reforms of the 2nd century BC, the legions took on the second, narrower meaning that is familiar in the popular imagination as close-order citizen heavy infantry.
At the end of the 2nd century BC Gaius Marius reformed the previously ephemeral legions as a professional force drawing from the poorest classes, enabling Rome to field larger armies and providing employment for jobless citizens of the city of Rome. However, this put the loyalty of the soldiers in the hands of their general rather than Rome itself. In this period all Italian regions obtained full Roman citizenship and provided a larger basis for the army, supplemented by poor urban Romans.
The legions of the Late Republic and Early Empire are often called Marian legions. Following the Battle of Vercellae in 101 BC, Marius granted all Italian soldiers Roman citizenship. He justified this action to the Senate by saying that in the din of battle he could not distinguish Roman from ally. This effectively eliminated the notion of allied legions; henceforth all Italian legions would be regarded as Roman legions, and full Roman citizenship was open to all the regions of Italy. Thus the three different types of heavy infantry were replaced by a single, standard type based on the Principes: armed with two heavy javelins called pila, the short sword called gladius, chain mail (lorica hamata) or banded armour (lorica segmentata), helmet and rectangular shield (scutum).
The role of allied legions would eventually be taken up by contingents of allied auxiliary troops, called Auxilia. Each legion had an auxilia of similar size, which contained specialist units, engineers and pioneers, artillerymen and craftsmen, service and support personnel and irregular units made up of non-citizens, mercenaries and local militia. These were usually formed into complete units such as light cavalry, light infantry or velites, and labourers. There was also a reconnaissance squad of 10 or more light mounted infantry called speculatores who could also serve as messengers or even as an early form of military intelligence service.
As part of the Marian reforms, the legions' internal organization was standardized. Each legion was divided into cohorts. Prior to this, cohorts had been temporary administrative units or tactical task forces of several maniples, even more transitory than the legions themselves. Now the cohorts were ten permanent units, composed of 6 and in the case of the first cohort 8 centuries each led by a centurion assisted by an optio, a soldier who could read and write. These came to form the basic tactical unit of the legions. The senior centurion of the legion was called the primus pilus, a career soldier and advisor to the legate that sometimes was promoted to the higher rank.
Every legion had a baggage train of 640 mules or about 1 mule for every 8 legionaries. To keep these baggage trains from becoming too large and slow, Marius had each infantryman carry as much of his own equipment as he could, including his own armour, weapons and 15 days' rations, for about 25-30 kg (50–60 pounds) of load total. To make this easier, he issued each legionary a forked stick to carry their loads on their shoulders. The soldiers were nicknamed Marius' Mules due to the amount of gear they had to carry themselves. This arrangement allowed for the supply train to become detached from the main body of the legion, thus greatly increasing the army's speed while on the march.
A typical legion of this period had 5,120 legionaries as well as a large number of camp followers, servants and slaves. Legions could contain as many as 6,000 fighting men when including the auxiliaries, although much later in Roman history the number was reduced to 1,040 to allow for greater mobility. Numbers would also vary depending on casualties suffered during a campaign; Julius Caesar's legions during his campaign in Gaul often only had around 3,500 men.
Tactics were slightly different from the past, but largely improved due to the professional training of the soldiers.
After the Marian reforms, and throughout the history of Rome's Late Republic, the legions played an important political role. By the 1st century BC the threat of the legions under a demagogue was recognized. Governors were not allowed to leave their provinces with their legions. When Julius Caesar broke this rule, leaving his province of Gaul and crossing the Rubicon into Italy, he precipitated a constitutional crisis. This crisis and the civil wars which followed brought an end to the Republic and led to the foundation of the Empire under Augustus in 27 BC.
For political and economic reasons, Augustus reduced the number of legions from nearly 50 at the end of his war against Mark Antony to only 28 which decomposed to 25 after the Battle of Teutoburg Forest in which 3 legions were slaughtered. Generals during the recent Republican civil wars had formed their own legions and numbered them as they wished. When these wars ended, Augustus was left with around fifty legions, with several double counts (multiple Legio Xs for instance). Beside streamlining the army Augusts also regulated the soldiers' pay. During this time, there was a high incidence of Gemina (twin) legions, where two legions were consolidated into a single organization (and was later made official and put under a dux and six duces).
At the same time, he greatly increased the number of auxiliaries to the point where they were equal in number to the legionaries. He also created the Praetorian Guard along with a permanent navy where served the liberti, or freed slaves.
Augustus' military policies proved sound and cost effective, and were generally followed by his successors. These emperors would carefully add new legions, as circumstances required or permitted, until the strength of the standing army stood at around 30 legions. With each legion having 5,120 legionaries usually supported by an equal number of auxiliary troops, the total force available to a legion commander during the Pax Romana probably ranged from 153,600 downwards, with the more prestigious legions and those stationed on hostile borders or in restive provinces tending to have more auxiliaries. Some legions may have even been reinforced at times with units making the associated force near 15,000–16,000 or about the size of a modern division.
Throughout the imperial era, the legions played an important political role. Their actions could secure the empire for a usurper or take it away. For example, the defeat of Vitellius in the Year of the Four Emperors was decided when the Danubian legions chose to support Vespasian.
In the empire, the legion was standardized, with symbols and an individual history where men were proud to serve. The legion was commanded by a legatus or legate. Aged around thirty, he would usually be a senator on a three year appointment. Immediately subordinate to the legate would be six elected military tribunes — five would be staff officers and the remaining one would be a noble heading for the Senate (originally this tribune commanded the legion). There would also be a group of officers for the medical staff, the engineers, record-keepers, the praefectus castrorum (commander of the camp) and other specialists such as priests and musicians.
In the Later Roman Empire, the number of legions was increased and the Roman Army expanded. There is no evidence to suggest that legions changed in form before the Tetrarchy, although there is evidence that they were smaller than the paper strengths usually quoted. The final form of the legion originated with the elite legiones palatinae created by Diocletian and the Tetrarchs. These were infantry units of around 1,000 men rather than the 5,000, including cavalry, of the old Legions. The earliest legiones palatinae were the Lanciarii, Joviani, Herculiani and Divitenses.
The 4th century saw a very large number of new, small legions created, a process which began under Constantine II. In addition to the elite palatinae, other legions called comitatenses and pseudocomitatenses, along with the auxilia palatina, provided the infantry of late Roman armies. The Notitia Dignitatum lists 25 legiones palatinae, 70 legiones comitatenses, 47 legiones pseudocomitatenses and 111 auxilia palatina in the field armies, and a further 47 legiones in the frontier armies. Legion names such as Honoriani and Gratianenses found in the Notitia suggest that the process of creating new legions continued through the 4th century rather than being a single event. The names also suggest that many new legions were formed from vexillationes or from old legions.
According to the late Roman writer Vegetius' De Re Militari, each century had a ballista and each cohort had an onager, giving the legion a formidable siege train of 59 Ballistae and 10 Onagers each manned by 10 libritors (artillerymen) and mounted on wagons drawn by oxen or mules. In addition to attacking cities and fortifications, these would be used to help defend Roman forts and fortified camps (castra) as well. They would even be employed on occasion, especially in the later Empire, as field artillery during battles or in support of river crossings.
All legionary soldiers would also receive a sizeable sum of money on the completion of their term of service: 3000 denarii from the time of Augustus and/or a plot of good farmland (good land was in much demand). Later, under Caracalla, the praemia increased to 5000 denarii.
In Gallic War (Bk IV, Para. 25), Julius Caesar describes an incident at the start of his first invasion of Britain in 55BC that illustrated how fear for the safety of the eagle could drive Roman soldiers. When Caesar's troops hesitated to leave their ships for fear of the Britons, the aquilifer of the tenth legion threw himself overboard and, carrying the eagle, advanced alone against the enemy. His comrades, fearing disgrace, 'with one accord, leapt down from the ship' and were followed by troops from the other ships.
With the birth of the Roman Empire, the legions created a bond with their leader, the emperor himself. Each legion had another officer, called imaginifer, whose role was to carry a pike with the imago (image, sculpture) of the emperor as pontifex maximus.
Each legion, furthermore, had a vexillifer who carried a vexillum or signum, with the legion name and emblem depicted on it, unique to the legion. It was common for a legion to detach some sub-units from the main camp to strengthen other corps. In these cases, the detached subunits carried only the vexillum, and not the aquila, and were called, therefore, vexillationes. A miniature vexillum, mounted on a silver base, was sometimes awarded to officers as a recognition of their service upon retirement or reassignment.
Civilians could also be rewarded for their assistance to the Roman Legions. In return for outstanding service, a citizen was given an arrow without a head.
The military discipline of the legions was quite harsh. Regulations were strictly enforced, and a broad array of punishments could be inflicted upon a legionary who broke them. Many legionaries became devotees in the cult of the minor goddess Disciplina, whose virtues of frugality, severity and loyalty were central to their code of conduct and way of life.