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 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 second 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.
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. 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:
- 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 3000 men, (plus the velites that normally enlarged the number to about 4200), the legion had only 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 couldn't 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 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 principes (sing. princeps) were men in their prime (late twenties to early thirties).
- 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 B.C.). 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. To fall upon the triarii was a Roman idiom - meaning to use one's last resort.
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.
Late Republic (107-30BC)
See also List of Roman legions for details of notable late Republican 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 (singular pilum), 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 because of 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 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 (30 BC-284 AD)
See Directory of Roman legions of the early Empire
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 Augustus 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.
Late Empire (from 284)
- See List of Roman legions for catalogue of late Empire legions
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 (Roman military), 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.