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The Roman legion was the largest military unit of the Roman army. A legion was roughly of brigade size, composed of 4,200 infantry and 300 cavalry in the republican period, extended to 5,200 infantry and 120 auxilia in the imperial period.
In the early Roman Kingdom legion may have meant the entire Roman army, but sources on this period are few and unreliable. The subsequent organization of legions varied greatly over time but legions were typically composed of around five thousand soldiers. During much of the republican era, a legion was divided into three lines of ten maniples. In the late republic and much of the imperial period (from about 100 BC), a legion was divided into ten cohorts, each of six (or five) centuries. Legions also included a small ala, or cavalry unit. By the third century AD, the legion was a much smaller unit of about 1,000 to 1,500 men, and there were more of them. In the fourth century AD, East Roman border guard legions (limitanei) may have become even smaller. In terms of organisation and function, the republican era legion may have been influenced by the ancient Greek and Macedonian phalanx.
Function and constitution
For most of the Roman Imperial period, the legions formed the Roman army's elite heavy infantry, recruited exclusively from Roman citizens, while the remainder of the army consisted of auxiliaries, who provided additional infantry and the vast majority of the Roman army's cavalry. (Provincials who aspired to citizenship gained it when honourably discharged from the auxiliaries.) The Roman army, for most of the Imperial period, consisted mostly of auxiliaries rather than legions.
Many of the legions founded before 40 BC were still active until at least the fifth century, notably Legio V Macedonica, which was founded by Augustus in 43 BC and was in Egypt in the seventh century during the Islamic conquest of Egypt.
Overview of typical organization and strength
Because legions were not permanent units 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. The republican legions were composed of levied men that paid for their own equipment and thus the structure of the Roman army at this time reflected the society, and at any time there would be four consular legions (with command divided between the two ruling consuls) and in time of war extra legions could be levied. Toward the end of the 2nd century BC, Rome started to experience manpower shortages brought about by property and financial qualifications to join the army. This prompted consul Gaius Marius to remove property qualifications and decree that all citizens, regardless of their wealth or social class, were made eligible for service in the Roman army with equipment and rewards for fulfilling 6 years of service provided by the state. The Roman army became a volunteer, professional and standing army which extended service beyond Roman citizens but also to non-citizens who could sign on as auxillia (auxiliaries) and were rewarded Roman citizenship upon completion of service and all the rights and privileges that entailed. In the time of Augustus, there were nearly 50 upon his succession but this was reduced to about 25–35 permanent standing legions and this remained the figure for most of the empire's history.
The legion evolved from 3,000 men in the Roman Republic to over 5,200 men in the Roman Empire, consisting of centuries as the basic units. Until the middle of the first century, ten cohorts (about 500 men) made up a Roman legion. This was later changed to nine cohorts of standard size (with six centuries at 80 men each) with the first cohort being of double strength (five double-strength centuries with 160 men each). By the fourth century AD, the legion was a much smaller unit of about 1,000 to 1,500 men, and there were more of them. This had come about as the large formation legion and auxiliary unit, 10,000 men, was broken down into smaller units - originally temporary detachments - to cover more territory. In the fourth century AD, East Roman border guard legions (limitanei) may have become even smaller. In terms of organisation and function, the Republican era legion may have been influenced by the ancient Greek and Macedonian phalanx.
The size of a typical legion varied throughout the history of ancient Rome, with complements of 4,200 legionaries and 300 equites (drawn from the wealthier classes – in early Rome all troops provided their own equipment) in the republican period of Rome (the infantry were split into 10 cohorts each of four maniples of 120 legionaries), to 5,200 men plus 120 auxiliaries in the imperial period (split into 10 cohorts, nine of 480 men each, plus the first cohort holding 800 men).
Roman kings (to c. 500 BC)
In the period before the raising of the legio and the early years of the Roman Kingdom and the Republic, forces are described as being organized into centuries of roughly one hundred men. These centuries were grouped together as required and answered to the leader who had hired or raised them. Such independent organization persisted until the 2nd century BC amongst light infantry and cavalry, but was discarded completely in later periods with the supporting role taken instead by allied troops. The roles of century leader (later formalized as a centurion), second in command and standard bearer are referenced in this early period.
Rome's early period is undocumented and shrouded in myths, but those myths tell that during the rule of Servius Tullius, the census (from Latin: censeō – accounting 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 their wealth and then organized into centuries as sub-units of the greater Roman army or legio (multitude). 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. These individuals would have had the most to lose should the state have fallen.
Roman Republic (509–107 BC)
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. In 494 BC, when three foreign threats emerged, the dictator Manius Valerius Maximus raised ten legions which Livy says was a greater number than had been raised previously at any one time.
Also, some warfare was still conducted by Roman forces outside the legionary structure, the most famous example being the campaign in 479 BC by the clan army of gens Fabia against the Etruscan city of Veii (in which the clan was annihilated). 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 each.
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 approximately ten cohorts (auxilia were not organized into legions) to support each Roman Legion.
In the middle of the Republic, legions were composed of the following units:
- Equites (cavalry): The cavalry was originally the most prestigious unit, where wealthy young Roman men displayed their skill and prowess, laying the foundation for an eventual political career. Cavalry equipment was purchased by each of the cavalrymen and consisted of a round shield, helmet, body armour, sword and one or more lances. The cavalry was outnumbered in the legion. In a total of circa 3,000 men, (plus the velites that normally enlarged the number to about 4,200), the legion only had around 300 horsemen, divided into 10 units (turmae) of 30 men. These men were commanded by decurions. In addition to heavy cavalry, there would be the light cavalry levied from poor citizens and wealthy young citizens not old enough to be in the hastati or the equites. In battle, they were used to disrupt and outflank enemy infantry formations and to fight off enemy cavalry. In the latter type of engagement, they would often (though not always) dismount some or all of the horsemen to fight a stationary battle on foot, an unusual tactic for the time, but one that offered significant advantages in stability and agility in a time before stirrups.
- Velites (light infantry): The velites were mainly poorer citizens who could not afford to equip themselves properly. Their primary function was to act as skirmishers – javelin-throwers, who would engage the enemy early in order either to harass them or to cover the movement of troops behind them. After throwing their javelins, they would retreat through the gaps between the maniples, screened from the attack of the enemy by the heavy infantry lines. With the shortage of cavalry in the army of the early to mid Republican army, the velites were also used as scouts. They did not have a precise formal organization or formation.
- Heavy infantry: This was the principal unit of the legion. The heavy infantry was composed of citizen legionaries that could afford the equipment composed of an iron helmet, shield, armour and pilum, a heavy javelin whose range was about 30 meters. After 387 BC, the preferred weapon for the hastati and principes was the gladius, a short sword. Their hobnailed sandals (caligae) were also an effective weapon against a fallen enemy. Prior to the Marian reforms (see below), the heavy infantry was subdivided, according to experience, into three separate lines of troops:
- The hastati (sing. hastatus) consisted of raw or inexperienced soldiers, considered to be less reliable than legionaries of several years' service. The Hastati were placed at the front for several reasons. One reason is the city of Rome could ill-afford to lose experienced soldiers, so they put the greenest soldiers at the front. If they survived, the Hastati/tus would gain invaluable experience. Another reason is if the newest soldiers succumbed to battle nerves and broke and tried to run, then there were experienced soldiers behind them to stiffen their resolve.
- The principes (sing. princeps) These were the more experienced soldiers, often better equipped than the Hastati, and having more experience on the battlefield, they would take up the second line in the battle in the event the Hastati failed or fled. They were the second wave in an early Republican Legion before the military reforms of Marius.
- The triarii (sing. triarius) were the veteran soldiers, to be used in battle only in extreme situations; they rested one knee down when not engaged in combat. The triarii served primarily as reserves or barrier troops designed to backstop the hastati and principes, and were equipped with long hastae (spears) rather than the pilum and gladius (the hastati and principes stopped using spears in 387 BC). Thus armed, they fought in a phalanx formation. The sight of an advancing armored formation of triarii legionaries frequently discouraged exultant enemies in pursuit of retreating hastati and principes troops. Ad triarios redisse – To fall back upon the triarii was a Roman idiom – meaning to use one's last resort.
Each of these three lines was subdivided into (usually 10) chief tactical units called maniples. A maniple consisted of two centuries and was commanded by the senior of the two centurions. At this time, each century of hastati and principes consisted of 60 men; a century of triarii was 30 men. These 3,000 men (twenty maniples of 120 men, and ten maniples of 60 men), together with about 1,200 velites and 300 cavalry gave the mid Republican ("manipular") legion a nominal strength of about 4,500 men.
Late Republic (107–30 BC)
The Marian reforms (of Gaius Marius) enlarged the centuries to 80 men, and grouped them into six-century "cohorts" (rather than two-century maniples). Each century had its own standard and was made up of ten units (contubernia) of eight men who shared a tent, a millstone, a mule and cooking pot.
Following the reforms of the general Marius in 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 the State of Rome itself. This development ultimately enabled Julius Caesar to cross the Rubicon with an army loyal to him personally and effectively end the Republic.
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. At the same time, 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 (singular pilum), the short sword called gladius, chain mail (lorica hamata), helmet and rectangular shield (scutum).
The role of allied legions would eventually be taken up by contingents of allied auxiliary troops, called Auxilia. Auxilia 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 centuries and in the case of the first cohort 5 double strength centuries each led by a centurion assisted by an optio. The cohorts came to form the basic tactical unit of the legions. Ranking within the legion was based on length of service, with the senior Centurion commanding the first century of the first cohort; he was called the primus pilus (First Spear), and reported directly to the superior officers (legates and tribuni). All career soldiers could be promoted to the higher ranks in recognition of exceptional acts of bravery or valour. A newly promoted junior Centurion would be assigned to the sixth century of the tenth cohort and slowly progressed through the ranks from there.
Every legion had a large baggage train, which included 640 mules (1 mule for every 8 legionaries) just for the soldiers' equipment. 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 cross stick to carry their loads on their shoulders. The soldiers were nicknamed Marius' Mules because of the amount of gear they had to carry themselves. This arrangement allowed for the possibility for the supply train to become temporarily detached from the main body of the legion, thus greatly increasing the army's speed when needed.
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 11,000 fighting men when including the auxiliaries. During the Later Roman Empire, the legion was reduced in size to 1,000 to allow for easier provisioning and to expand the regions under surveillance. 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 not very different from the past, but their effectiveness was largely improved because of 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.
Early Empire (27 BC–AD 284)
Generals, during the recent Republican civil wars, had formed their own legions and numbered them as they wished. 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 legatus and six duces). At the end of the civil war against Mark Antony, Augustus was left with around fifty legions, with several double counts (multiple Legio Xs for instance). For political and economic reasons, Augustus reduced the number of legions to 28 (which diminished to 25 after the Battle of Teutoburg Forest, in which 3 legions were completely destroyed by the Germanics).
Beside streamlining the army, Augustus also regulated the soldiers' pay. 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. The legions also became permanent at this time, and not recruited for particular campaigns. They were also allocated to static bases with permanent castra legionaria (legionary fortresses).
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 (hence the wry remark of the philosopher Favorinus that It is ill arguing with the master of 30 legions). With each legion having 5,120 legionaries usually supported by an equal number of auxiliary troops (according to Tacitus), the total force available to a legion commander during the Pax Romana probably ranged from 11,000 downwards, with the more prestigious legions and those stationed on hostile borders or in restive provinces tending to have more auxiliaries. By the time of the emperor Severus, 193-211, the auxiliaries may have composed 55 to 60% of the army, 250,000 of 447,000. 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.
Later Empire (from 284 AD)
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 palatini, 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. In addition, there were 24 vexillationes palatini, 73 vexillationes comitatenses; 305 other units in the Eastern limitanei and 181 in the Western limitanei. A rare instance of apparent direct continuity between the legions of the early Empire and those of the post-6th century army was Legion V Macedonica; created in 43 BC, recorded in the Notitia Dignitatum as a legione comitatense under the title of Quinta Macedonica and surviving in Egypt until the Arab conquest of 637 AD.
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.
Despite a number of organisational changes, the Legion system survived the fall of the Western Roman Empire. It was continued within the Eastern Roman Empire until the 7th century, when reforms begun by Emperor Heraclius to counter the increasing need for soldiers resulted in the Theme system. Despite this, the Eastern Roman/Byzantine armies continued to be influenced by the earlier Roman legions, and were maintained with similar levels of discipline, strategic prowess, and organization.
Aside from the rank and file legionary (who received the base wage of 10 assēs a day or 225 denarii a year), the following list describes the system of officers which developed within the legions from the Marian reforms (104 BC) until the military reforms of Diocletian (c. 290).
- Legatus Augusti pro praetore, Imperial Legate: The commander of two or more legions. The Imperial Legate also served as the governor of the province in which the legions he commanded were stationed. Of Senatorial rank, the Imperial Legate was appointed by the Emperor and usually held command for 3 or 4 years. In the present time, an Imperial Legate would be called a General.
- Legatus legionis, Legion Legate: The overall legion commander. The post was usually filled by a senator, appointed by the emperor, who held command for 3 or 4 years, although he could serve for a much longer period. In a Roman province with only one legion, the legatus was also the provincial governor. In such circumstances, the Legatus was dual-hatted as both Legion Legate and Imperial Legate. The Legion Legate also served as commander of the auxiliary units attached to the legion though they were not formally a part of the legion's command structure.
- Tribunus laticlavius, Broad Band Tribune: Named for the broad striped tunic worn by men of senatorial rank, this tribune was appointed by the emperor or the Senate. Though generally young, he was more experienced than the tribuni angusticlavii, he served as second in command of the legion, behind the legate. Because of his age and inexperience he was not the actual second in command in battle, but if the legate died he would take command of the legion.
- Praefectus castrorum, Camp Prefect: The Camp Prefect was third in command of the legion. Generally he was a long serving veteran from a lower social status than the tribunii whom he outranked, and who previously had served as primus pilus and finished his 25 years with the legions. He was used as a senior officer in charge of training a legion, though he could also command a cohort of auxiliaires.
- Tribuni angusticlavii, Narrow Band Tribunes: Each legion had five lower ranking tribunes, who were normally from the equestrian class and had at least some years of prior military experience. They often served the role of administrative officers. This tribunate was often a first, but optional, step in a young man's political career (see cursus honorum).
The rank of centurion was an officer grade that included many ranks, meaning centurions had very good prospects for promotion. The most senior centurion in a legion was known as the primus pilus (first file or spear), who directly commanded the first century of the first cohort and commanded the whole first cohort when in battle. Within the second to tenth cohorts, the commander of each cohort's first century was known as a pilus prior and was in command of his entire cohort when in battle. The seniority of the pilus prior centurions was followed by the five other century commanders of the first cohort, who were known as primi ordines.
The six centuries of a normal cohort, were, in order of precedence:
- The forward hastati (forward spears)
- The rear hastati (rear spears)
- The forward principes (forward principal line)
- The rear principes (rear principal line)
- The forward triarii (forward third line)
- The rear triarii (rear third line)
The centuries took their titles from the old use of the legion drawn up in three lines of battle using three classes of soldier. (Each century would then hold a cross-section of this theoretical line, although these century titles were now essentially nominal.) Each of the three lines is then sub-divided within the century into a more forward and a more rear century.
- Primus pilus, literally First File: The Primus Pilus was the commanding centurion of the first century, first cohort and the senior-most centurion of the entire legion. (Unlike other cohorts, the first cohort had only one javelin century, instead of a "front spear" and a "back spear" century.) The primus pilus had a chance of later becoming a Praefectus Castrorum. When the primus pilus retired, he would most likely gain entry into the equestrian class. He was paid 60 times the base wage. Primus Pilus were also paid more than an average centurion and like a narrowband tribune.
- Pilus prior: The "front file" centurions were the commanders of the 10 1st centuries within the legion, making them senior centurions of their respective cohorts. While the legion was in battle formation, the Pilus Prior was given command of their entire cohort. The Primus Pilus was also a Pilus Prior, and the most senior of all the centurions within the legion. These positions were usually held by experienced veteran soldiers who had been moved up within the ranks. This rank is subordinate to the Primus Pilus.
- Primi ordines: The "ranks of the first [cohort]" were the five centurions of the first cohort, and included the primus pilus. They, excluding the primus pilus, were paid 30 times the base wage. This rank is senior to all other centurions, save the primus pilus and pilus prior.
- Other centurions: Each legion had 59 or 60 centurions, one to command each century of the 10 cohorts. They were the backbone of the professional army and were the career soldiers who ran the day-to-day life of the soldiers and issued commands in the field. They were generally moved up from the ranks, but in some cases could be direct appointments from the emperor or other higher-ranking officials. The cohorts were ranked from the first to the tenth and the century within each cohort ranked from 1 to 6, with only five centuries in the first cohort (for a total of 59 centurions and the primus pilus). The century that each centurion commanded was a direct reflection of his rank: command of the 1st century of the first cohort was the highest, and the 6th century of the 10th cohort was the lowest. Paid ten times the basic wage.
- Optio: One for each centurion (59–60), they were appointed by the centurion from within the ranks to act as his second in command and were paid twice the basic wage. Equivalent to a modern First Sergeant.
- Tesserarius: (Guard commander) One for each century. They acted as seconds to the Optios and were paid one and a half times the basic wage. Keeper of the watchword, administrative assistant to HQ Staff, third in command of a century. These men fought as normal soldiers when the century they were attached to was not in the vanguard. Equivalent to a modern Staff Sergeant.
- Decurion: Commanded a cavalry unit (turma) of 10 to 30 eques legionis.
- Decanus: Commanded a contubernium or ten men tent party, eight soldiers and two non-combatants, equivalent to a Sergeant or Corporal. A group of four soldiers would be referred to as a Quaternion.
Special duty posts
- Aquilifer: A single position within the legion. The aquilifer was the legion's standard– or Aquila (eagle)– bearer and was an enormously important and prestigious position. Losing the aquila was considered the greatest dishonor a legion could endure. This post therefore had to be filled with steady veteran soldiers, with an excellent understanding of the tactics of the legion. He was paid twice the basic wage.
- Signifer: Each century had a signifer (thus, there were 59 in a legion) and within each cohort the 1st century's signifer would be the senior. He was standard-bearer for the centurial signum, a spear shaft decorated with medallions and topped with an open hand to signify loyalty, which was a rallying point for the soldiers. In addition to carrying the signum, the signifer also assumed responsibility for the financial administration of the unit and functioned as the legionaries' banker. He was paid twice the basic wage.
- Cornicen (Horn blower): Worked hand in hand with the signifer drawing the attention of the men to the centurial signum and issuing the audible commands of the officers. He was paid twice the basic wage.
- Imaginifer: A special position from the time of Augustus onwards. Carried the standard bearing the image of the Emperor as a constant reminder of the troops' loyalty to him. He was paid twice the basic wage.
- Immunes: Immunes were legionary soldiers who possessed specialized skills, qualifying them for better pay and excusing them from labour and guard work. Engineers, artillerymen, musicians, clerks, quartermasters, drill and weapons instructors, carpenters, hunters, medical staff and military police were all immune soldiers. These men were still fully trained legionaries, however, and were called upon to serve in the battle lines when needed.
- Evocatus: A veteran of the Roman army who had earned his military diploma for military service, but had chosen to re-enlist. They received double pay and were excluded from regular duties, such as manual labour.
- Duplicarius: A special pay grade that could also be awarded regardless of rank, received double the basic pay.
From the time of Gaius Marius onwards, legionaries received 225 denarii a year (equal to 900 Sestertii); this basic rate remained unchanged until Domitian, who increased it to 300 denarii. In spite of the steady inflation during the 2nd century, there was no further rise until the time of Septimius Severus, who increased it to 500 denarii a year. However, the soldiers did not receive all the money in cash, as the state deducted a clothing and food tax from their pay. To this wage, a legionary on active campaign would hope to add the booty of war, from the bodies of their enemies and as plunder from enemy settlements. Slaves could also be claimed from the prisoners of war and divided amongst the legion for later sale, which would bring in a sizeable supplement to their regular pay.
All legionary soldiers would also receive a praemia (veterans' benefits) on completion of their term of service of 25 years or more: a sizeable sum of money (3,000 denarii from the time of Augustus) and/or a plot of good farmland (good land was in much demand); farmland given to veterans often helped in establishing control of the frontier regions and over rebellious provinces. Later, under Caracalla, the praemia increased to 5,000 denarii.
From 104 BC onwards, each legion used an aquila (eagle) as its standard symbol. The symbol was carried by an officer known as aquilifer, and its loss was considered to be a very serious embarrassment, and often led to the disbanding of the legion itself. Normally, this was because any legion incapable of regaining its eagle in battle was so severely mauled that it was no longer effective in combat.
In Gallic War (Bk IV, Para. 25), Julius Caesar describes an incident at the start of his first invasion of Britain in 55 BC 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. This was considered a great honour and would bring the recipient much prestige.
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.
- Castigatio – being hit by the centurion with his staff or animadversio fustium (Tac. Annals I, 23)
- – Reduction of rations or to be forced to eat barley instead of the usual grain ration
- Pecuniaria mulcta – Reduction in pay, fines or deductions from the pay allowance
- – Flogging in front of the century, cohort or legion
- – Whipping with the flagrum (flagellum, flagella), or "short whip" – a much more brutal punishment than simple flogging. The "short whip" was used by slave volunteers, volones, who constituted the majority of the army in the later years of the Roman Empire.
- Gradus deiectio – Reduction in rank
- Missio ignominiosa – Dishonourable discharge
- – Loss of time in service advantages
- Militiae mutatio – Relegation to inferior service or duties.
- Munerum indictio – Additional duties
- Fustuarium – a sentence for desertion or dereliction of duty. The legionary would be stoned or beaten to death by cudgels, in front of the assembled troops, by his fellow soldiers or those whose lives had been put in danger. Soldiers under sentence of fustuarium who escaped were not pursued but lived under sentence of banishment from Rome. In the event that a group of legionaries are to be subjected to this punishment, the Tribune would make an alteration in order to spare the majority of the accused. The Tribune would first select a handful of the guilty men, and those selected would be condemned to the original penalty under the Fustuarium. The remainder of the accused would then be driven out of the camp and forced to live in an undefended location for a chosen period of time; they were also limited to eating only barley. 
- Decimation – According to 17th century belief[failed verification](possibly folk etymology), the Romans practiced this punishment in which a sentence was carried out against an entire unit that had mutinied, deserted, or shown dereliction of duty. One out of every ten men, chosen by lots, would be beaten to death, usually by the other nine with their bare hands, who would be forced to live outside the camp and in some instances obliged to renew the military oath, the sacramentum.
Factors in the legion's success
Montesquieu wrote that "the main reason for the Romans becoming masters of the world was that, having fought successively against all peoples, they always gave up their own practices as soon as they found better ones."
Examples of ideas that were copied and adapted include weapons like the gladius (Iberians) and warship design (cf. Carthaginians' quinquereme), as well as military units, such as heavy mounted cavalry and mounted archers (Parthians and Numidians).
- Roman organization was more flexible than those of many opponents. Over time, the legions effectively handled challenges ranging from cavalry, to guerrillas, and to siege warfare.
- Roman discipline (cf. decimation (Roman army)), organization and systematization sustained combat effectiveness over a longer period. These elements appear throughout the legion in training, logistics, field fortification etc.
- The Romans were more persistent and more willing to absorb and replace losses over time than their opponents. Wars with Carthage and the Parthians and most notably, the campaigns against Pyrrhus of Epirus, illustrate this.
- Roman leadership was mixed, but over time it was often effective in securing Roman military success.
- The influence of Roman military and civic culture, as embodied particularly in the heavy infantry legion, gave the Roman military consistent motivation and cohesion.
- Strict, and more importantly, uniform discipline made commanding, maintaining, and replacing Roman legionaries a much more consistent exercise.
- Roman military equipment (cf. Roman military personal equipment), particularly armor, was more withstanding and far more ubiquitous, especially in the late Republican and Early Imperial era, than that of most of their opponents. Soldiers equipped with shields, helmets and highly effective body armor had a major advantage over warriors protected, in many cases, with nothing other than their shields, particularly in a prolonged engagement.
- Roman engineering skills were second to none in ancient Europe, and their mastery of both offensive and defensive siege warfare, specifically the construction and investiture of fortifications (cf. sudis, castra), was another major advantage for the Roman legions.
- Roman military training focused on the more effective thrusting of the sword rather than the slash.
- Auxiliaries (Roman military)
- Early Roman army
- Imperial Roman army
- Late Roman army
- List of Roman wars
- List of Roman battles
- List of topics related to ancient Rome
- List of Roman legions
- List of Roman auxiliary regiments
- Military history of ancient Rome
- Marian Roman army
- Roman army
- Roman army of the mid-Republic
- Roman military personal equipment
- Structural history of the Roman military
- For a more detailed analysis, as well as the Romans in battle, see the articles Roman infantry tactics and Roman military personal equipment.
- Cornell, T. J. (1995): The Beginnings of Rome
- Manousos Kambouris, Spyros Bakas (2016). "Greco-Macedonian influences in the manipular Legion system". Archaeology and Science. 11 2015: 145–154.
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- Edwards, H. J. "The Histories of Polybius: Fragments of Book VI p.357". The University of Chicago. Retrieved April 2, 2020.
- "It's totally fine to use 'decimate' as a synonym for 'devastate'. This is why". Merriam-Webster. Retrieved October 24, 2018.
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