The Roman legion (Latin: legiō, .mw-parser-output .IPA-label-smallfont-size:85%.mw-parser-output .references .IPA-label-small,.mw-parser-output .infobox .IPA-label-small,.mw-parser-output .navbox .IPA-label-smallfont-size:100%Latin: [ˈɫɛɡioː]), the largest military unit of the Roman army, was composed of Roman citizens serving as legionaries. During the Roman Republic the manipular legion comprised 4,200 infantry and 300 cavalry. After the Marian reforms in 107 BC the legions were formed of 5,200 men and were restructured around 10 cohorts, the first cohort being double strength. This structure persisted throughout the Principate and middle Empire, before further changes in the fourth century resulted in new formations of around 1,000 men.
A legion had 4,800 legionaries (in 10 cohorts of 6 centuries of 80 legionaries) from the late republic to the time of Julius Caesar. It expanded to 5,280 men plus 120 auxiliaries in the Imperial period (split into 10 cohorts, nine of 480 men each, with the first cohort being almost double-strength at 800 men). These are typical field strengths while "paper strength" was slightly higher (e.g. 600 and 1,200 respectively for Imperial cohorts).
In the early Roman Kingdom the term legion may have meant the entire Roman army, but sources on this period are few and unreliable.[2] The subsequent organisation 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, each 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 organization and function, the republican era legion may have been influenced by the ancient Greek and Macedonian phalanx.[3]
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 cavalry (provincials who aspired to Roman 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.[4]
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.
On the other hand, Legio XVII ("Seventeenth Legion"), Legio XVIII ("Eighteenth Legion") and Legio XIX ("Nineteenth Legion"), founded by Augustus around 41 BC,[5][6][7] were destroyed by a Germanic alliance led by Arminius in the Varian Disaster (September 9, AD 9) and never raised again by the Romans thereafter.[8]
The legions of the Republic were only conscripted in times of conflict and usually limited to four legions, two to be commanded by each consul, though more could be levied if needed. Legionaries lacked the opportunity of a military career; they were not paid well, their primary form of income being what they could loot from the battlefield, and were simply called upon when needed and returned to their civilian lives when they were no longer required.
After a crushing defeat at the Battle of the Allia, in 387 BC the military structure was reformed. Under the Camillan system the legions were initially structured based on social class, with the poorest being the first line of the formation. The legionaries most often fought with hastae (spears) and scuta (large rectangular shields) in a checkered maniple formation with assistance from skirmishers. The exception to this was the triarii, the final line of the formation who instead fought as hoplites, using Greek clipei and whose wealth could afford them gladii in the case of a broken spear.
The Republican 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 AD, ten cohorts 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).
In the period before the raising of the legio and the early years of the Roman Kingdom and the Roman Republic, forces are described as being organised into centuriae 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 organisation 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 formalised as a centurion), second in command and standard bearer are referenced in this early period.
At some point after the overthrow of the Roman monarchy 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.[11]
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 organised 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 organised into legions) to support each Roman Legion.
The role of allied legions would eventually be taken up by contingents of allied auxiliary troops, called auxilia. Auxilia contained immunes (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 ten or more light mounted infantry called speculatores, who could also serve as messengers or even as an early form of military intelligence service.
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.
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 recognised. Roman 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.
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 organisation (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 three 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 Roman 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).
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 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.[citation needed]
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