The Roman legion (from Latin legio "military levy, conscription," from legere — "to choose") is a term that can apply both as a translation 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 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 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.
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 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. Early Roman centuries typically had a leader (later formalised as a centurion), a second in command and a standard bearer.
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 trumpeteers. 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 strangely 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 a plain. Spearmen would deploy themselves 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. 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.
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.) 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.