Roman legion

A Roman legion (from Latin "legio" "military levy, conscription", from "legere" "to choose") was the largest unit of the Roman army involving from 3000 men in early times to over 5200 men in imperial times, consisting of "centuries" as the basic units. Until the middle of the first century, 10 cohorts (about 5,000 men) made up a Roman Legion. This was later changed to nine cohorts of standard size (with 6 centuries at 80 men each) and one cohort, the first cohort, of double strength (5 double-strength centuries with 160 men each).
In the early Roman Kingdom the "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, divided during the republican era into three lines of ten maniples, and from about 100 BC into ten cohorts. 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.
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.
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 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 that 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.
A legion 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 recruitment of non-citizens was rare but appears to have occurred in times of great need; For example, Caesar appears to have recruited the Legio V Alaudae mostly from non-citizen Gauls.
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).
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 formalised as a centurion), second in command and standard bearer are referenced in this early period.
Much Roman history of the era is shrouded in legend, but it is believed that during the reign 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 organised 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.
The first and wealthiest common 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 two were trumpeters. Roman soldiers had to purchase their own equipment. The second and third class also acted as spearmen but were less heavily armoured and carried a larger oval or rectangular shield. The fourth class could afford no armour; perhaps bearing a small shield and 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 centuries raised from this class, two of which were designated engineers. The army officers as well as the cavalry were drawn from leading citizens who enrolled as equestrians ("equites"). The equites were later placed in smaller groups of 30 that were commanded by decurions (which means commander of ten). There were 18 centuries of equites.
Until the 4th century BC, the massive Greek phalanx was the mode of battle. Roman soldiers would have thus looked much like Greek "hoplites". Tactics were no different from those of the early Greeks and battles were joined on flat terrain. Spearmen would deploy in tightly packed rows to form a shield wall with their spears pointing forwards. They charged the enemy supported by javelin throwers and slingers; the cavalry pursued the enemy, sometimes dismounting to support infantry in dire situations. The phalanx was a cumbersome military unit to manoeuvre and was easily defeated by mountain tribes, such as the Volsci or Samnites, in rough terrain.
Early civilian authorities called "praetors" doubled as military leaders during the summer war season. A declaration of war included a religious ceremony ending with 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. 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:
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.
"See also List of Roman legions for details of notable late Republican legions"