The Roman legion (from the Latin legio, meaning levy) was the basic military unit of ancient Rome. It consisted of about 5,000 to 6,000 (later 8,000) infantry soldiers and several hundred auxiliary cavalrymen and ranged troops, typically skirmishers. Legions were named and numbered; about 50 have been identified. Usually there were 28 Legions plus their Auxiliaries, with more raised as needed or as able.

Due to the enormous military successes of the Roman Republic and the Roman Empire the legion has long been regarded as the prime ancient model for military efficiency and ability.

Early history

Originally, in the time of the Kings, the legio was the whole Roman army, comprised of levied citizens. Much of Roman history during this time is based on legends, but it is believed that during the reign of Servius Tullius , all Roman able-bodied, property-owning male citizens were first divided into five classes for military service based on wealth, since soldiers provided their own weapons and equipment. These classes were further orgainized into units of 100 called Centuries. At some point, possibly in the beginning of the Roman Republic, 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 the warfare was mostly concentrated in raids, it is uncertain if the full manpower of the legions was summoned at one time. Legions become organized in a more formal way in the 4th century BC, as Roman warfare evolved to more frequent and planned operations, and the consular army was raised to two legions. The military tribunes appeared after 331 BC (At first these tribunes took turns as the legion's commanding officer). The internal organization of the legion became more sophisticated, from the classic phalanx to the manipular system, and allowed important tactical innovations. For the first time, the classes of soldiers who comprised the legions was based on experience and age rather than wealth, with standard weapons and equipment issued by the state. (The exception was the Equites, who comprised the cavalry elements and still provided their own horses.) Later in the Roman Republic, each legion was commonly reinforced by a legion of allied troops, called Alla.

Organization

In the Republic, legions had an ephemeral existence. Except for Legio I to IV, which were the consular army (two per consul), other units were levied by campaign. Rome's Italian allies were required to provide a legion to support each Roman legion. However, at the end of the 2nd century BC Marius reformed the legions to be a professional force drawing from the poorest classes, enabling Rome to field larger armies and providing employment for jobless citizens. However this put the loyalty of the soldiers in the hands of their general rather than Rome itself, and after several civil wars the Republic was abolished. 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 legate or legatus. 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 and the praefectus castrorum (commander of the camp) as well as other specialists such as priests and musicians.

In the middle of the Republic, legions were composed of the following units:

Cavalry or equites
The cavalry was originally the most prestigious unit, where wealthy young Roman upstarts 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 javelins. The cavalry was outnumbered in the legion. In a total of circa 3000 men, the legion had only around 300 horsemen, divided into 10 units of 30 men. These men were commanded by decurions. Allied legions were required to have 600 horsemen. 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.

Light infantry or velites
The velites were basically javelin throwers who did not have a precise formal organization or function in battle, being used where there was need for them. Normally they would deploy in front of the legion and try to break up the enemy formation, though this rarely accomplished much. After throwing their javelins they would retreat through the gaps between the maniples.

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 bronze helmet, shield, armour and short spear (pilum). The preferred weapon was the gladius, a short sword. The heavy infantry was subdivided, according to the legionaries' experience in the Republican Legion prior to the Marian reforms, which abolished the separate classes of troops turning the legion into a professional force, into three separate lines:

The hastati (sing. hastatus) were the younger ones and formed the front line.
The principes (sing. princeps), men in their prime (late twenties early thirties), composed the second line of the legion.
The triarii (sing. triarius) were the veteran soldiers that occupied the rear; only in extreme situations would they be used in battle. They were equipped with spears rather than the pilum and gladius.

Each of these three lines was subdivided into maniples, the lowest subunit of the army, each consisting of two centuries commanded by the senior of the two centurions. Centuries were nominally 80 soldiers each (not 100, as is popularly believed), but in practice might be as few as 60, especially in the less numerous triarii maniples. 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.

During deployment, the maniples were commonly arranged in a chequered formation called quincunx. However, it is unlikely that they entered battle in this way. Principes maniples would cover the open space left by the hastati, and be covered in return by triarii maniples. The two centuries of each maniple were formed up one behind the other. After the velites had retreated through the 'Hastati', the 'posterior' century would march to the left and then forward so that they presented a solid line. Then the Hastati would charge. If they were losing the fight, the 'posterior' century returned to its position creating gaps again. Then the maniples would fall back through the gaps in the 'Principes', who followed the same procedure to form a battle line and charge. If the Principes could not break the enemy, they would retreat behind the 'Triarii' and the whole army would leave the battlefield in good order. This is only standard procedure and was often modified; at Zama, Scipio deployed his entire legion in a single line to envelop Hannibal's army just as Hannibal had done at Cannae.

Marian Legions

In the late republic, the cohort of which there were six to ten, became the basic tactical unit. The cohort is composed of five to eight centuries and is led by a centurion assisted by an optio, a soldier who could read and write. The senior centurion of the legion was called the primus pilus, a career soldier and advisor to the legate. Under the Marian (named after Gaius Marius) reforms, the three different types of heavy infantry were replaced by a single, standard type of Legionary based on the Principes. The Legions were organized into permanent Cohorts for the first time. Prior to this Cohorts were temporary administrative units or tactical task forces of several maniples. Their existence was even more ephemeral than that of the legions of the early republic themselves. The Cohort legions of the late republic and early empire are often called Marian legions. Following the Battle of Vercellae in 101 B.C. Marius granted all Italian soldiers Roman citizenship. He justified this action to the Senate by saying 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. The role of allied legions would eventually be taken up by auxiliaries. Every legion had a baggage train of 500-550 mules, or about 1 mule for every 10 legionaries. To keep these baggage trains from becoming too large, Marius had each man carry as much of his own equipment as he could, including his own armor, weapons and 15 days' rations or about 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 nicknamed these sticks Marius' Mules.

A typical legion of this period had around 4-5000 legionaries as well as a large number of camp followers, servants and slaves. Legions could contain as many as 6,000 fighting men, although at times in Roman history the number was reduced to 1,000 to curb the power of mutinous commanders. Julius Caesar's legions had only around 3,500 men.

Auxiliaries, each Legion had a same size or near same size auxiliary which contained specialist units, engineers and pioneers, artillerymen and siege craftsmen, service and support units plus units made up of non-citizens (who were granted Roman citizenship upon discharge) and undesireables. These were usually formed into complete units such as light cavalry, light infantry or velites, and laborers. 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.

Siege artillery According to Vegetius' De Re Militari, each Century had a Ballista and each Cohort had an Onager (or Wild Ass in Latin). 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.

Legionary Officers

(Note the following list describes the system of officers which developed within the legions from the Marian reforms (circa 105 B.C.E.) until the military reforms of Diocletian (circa 290).

Senior Officers

• Legatus Legionis: The overall Legionary commander. This post was generally appointed by the emperor to a man who was a former Tribune and held command for 3 or 4 years, although could serve for a much longer period. In a province with only one legion, the Legatus was also the provincial governor and in provinces with multiple legions, each legion has a Legatus and the provincial governor has overall command of them all. The office is similar to the modern Major General.
• Tribunus Laticlavius: Named for the broad striped toga worn by men of senatorial rank, this tribune was appointed by the Emperor or the Senate. Though generally quite young and less experienced than the Tribuni Angusticlavii, he served as second in command of the legion, behind the Legate. The office is similar to the modern Brigadier General.
• Praefectus Castrorum: The camp Prefect. Generally he was a long serving veteran who had been promoted through the ranks of the centurions and was 3rd in overall command. The rank is similar to the modern Colonel.
• Tribuni Angusticlavii: Each legion had 5 military tribunes of equestrian (knight) class citizens. They were in many cases career officers and served many of the important administrative tasks of the Legion, but still served in a full tactical command function during engagements. The rank is similar to the modern Lieutenant Colonel.
• Primus Pilus: The "First File" or "first spearman" was the commanding centurion of the first cohort and the senior centurion of the entire Legion. When the Primus Pilus retired he was guaranteed entry into the Equestrian Class. He was paid 60 times the base wage. The rank is similar to a modern Major, but of the highest degree.

• 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 as well as issuing 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 5 Century 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 First Century of the First Cohort was the highest and the 6th Century of the 10th Cohort was the lowest). The rank is compared to the modern Captain.
• Primi Ordines: The 5 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 similar to the modern Captain, but senior to all other Centurions, save the Primus Pilus and Pilus Prior.
• The Pilus Prior: The 10 Centurions of the First Centuires of each of the 10 Cohorts were called Pilus Prior and were the most senior Centurion within their respective cohort. While the legion was in battle formation, the Pilus Prior was given command of the entire cohort, with each of the Centurions as his lieutenants. The Primus Pilus was also a Pilus Prior, only the most senior of all the Centurions within the Legions. These positions were usually held by experienced veteran soldiers who had been moved up within the ranks. This rank is similar to the modern Major, but subordinate to the Primus Pilus.

• Optio: One for each Centurion (59), they were appointed by the Centurion from within the ranks to act as his second in command. Paid twice the basic wage. This rank is similar to the modern First Lieutenant.
• Tesserarius: (Guard Commander) Again there were 59 of these, or one for each Century. They acted seconds to the Optios. Paid one and a half times the basic wage. This rank is similar to the modern Second Lieutenant.
• Decurion: Led a Contubernium, or "Tent Group", of 8 to 10 men. Each Century had 8 to 10 of these. Paid one and a quarter times basic wage. This rank is similar to the Non-commissioned officer Sergeant.
• Aquilifer: A single position within the Legion. The Aquilifer was the Legion's Standard or Eagle bearer and was an enormously important and prestigious position. Losing the aquila (eagle) was considered a great dishonor. The next step up would be a post as a Centurion. Paid twice the basic wage.
• Signifer: Each Century had a Signifer (59). He was responsible for the men's pay and savings, and the standard bearer for the Centurial Signum, a spear shaft decorated with medallions and often topped with an open hand to signify the oath of loyalty taken by the soldiers. It was this banner that the men from each individual Century would rally around. A soldier could also gain the position of Discentes signiferorum, or standard bearer in training. Paid twice the basic wage.
• Cornicen: (Horn blower) They 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.
• Imaginifer: Carried the Standard bearing the image of the Emperor as a constant reminder of the troop's loyalty to him.