The Roman Army was a military organization employed by the Roman Kingdom, Roman Republic and later Roman Empire as part of the Roman military. Its main infantry constituent for much of its history was the Roman legion; (for a catalogue of individual legions, dates and deployments, see List of Roman legions). The army in the late Roman Empire consisted of about 375,000 men, organized hierarchically. The main Roman soldiers in the Empire were the legionaries. There were other soldiers in the army known as the auxilia. Auxilia were non-citizens recruited mostly from the provinces. They were paid less than legionaries but at the end of their service they were granted Roman citizenship (see Roman auxiliaries). Despite the fact that the Roman Navy controlled the Mediterranean, it never operated as an entity independent from the Roman Army.
The army was enmeshed with Roman political life. Commanders of legions, or Legatus Legionis were appointed by the emperor and the tribunes and other senior officers were senate appointments. Centurions, the professional soldiers commanding units of 80-100 men, were often promoted from the ranks by the emperor or other influential political figures.
Among Roman soldiers, the smallest organization unit was called a "contubernium". This was a group of 8 soldiers (however originally it was made of 10), that shared a tent and ate together. There were 10 contubernia in a "century". A century was the next largest group of soldiers. A century was a group of originally 100 men in the Early Roman Republic but later reduced to 80 men during the Roman Empire. The next largest group of soldiers were called "maniples". Next were the "cohorts". These were made up of 6 centuries (480 men). A "prima cohors" was the first cohort in a legion; it was much larger than the other cohorts, containing about 5 double strength centuries (800-men). Finally, the largest group in the Roman Army was the legion.There were ten cohorts including the "prima cohors" in a legion. A full-strength legion contained 6,000 men though it was not uncommon for most legions to be undermanned due to previous battles. All of these numbers depended on the date (ex. Scipio Africanus reformation, Gaius Marius reformation). The republican army's strength, in peace, was four legions, but the number was increased during wartime. The highest number of legions was 70 after the civil war between Octavian (Augustus) and Mark Antony, due to having two whole Roman empires fighting when the remainder of Antony's forces joined with Octavian's. The number was decreased to 28 legions soon after, as the economically strained empire could not pay such huge numbers. After the Varus disaster, only 25 legions remained.
Weapon and Equipment Main article: Roman military personal equipment In an early to mid-Republican era legionaries usually bought their own gear. Hastati, the first line, usually had breastplates and occasionally wore lorica hamata, or chainmail. The wealthier principes could afford lorica hamata but they were sometimes seen wearing the cheaper cuiriasses. Both hastati and principes were each armed with a gladius - a short, 60 centimeter sword - and each had two pila (javelins). The Triarii's primary weapon was the hasta, a 2 meter long spear. They were also armed with the gladius and had an early form of the lorica segmenta. All legionaries had a large rectangular shield (scutum) which had rounded corners. By the late Republican period, all legionaries carried a gladius, two pila, a new, larger version of the scutum, and wore chainmail. Lorica segmenta, or the iron band armor, was in sporadic use in the early 1st century  but commonly worn in the next two centuries.
A set of Roman armor would include one of a variety of body armor types (usually designed to be flexible but strong; a centurion's body armor differs from that of the legionary), a shielders and turtlesith a special design/decoration for each legion), leggings or greaves, an apron (for decoration and protecting the groin, mostly made of metal), marching sandals called Caligae (with studs on the sole), a coarse woolen tunic, a belt (showing a soldier's position/rank in the army), and lastly a helmet called Galea (with cheek, ear and neck protection). A helmet might have also held a crest if the Roman was an officer or of higher rank than a peer.
Personal armor The lorica hamata is a type of chainmail crephole armor used during the Roman Republic and Roman Empire as a standard-issue armor for both the legionaries (higher quality version of the lorica hamata) and secondary troops (Auxilia). The lorica segmentata was a type of armor introduced in the early 1st century AD. The armor itself consisted of broad ferrous (iron) strips ('girth hoops') fastened to internal leather straps. The lorica squamata was a type of scale armor used during the Republic and at later periods. The Scutum, (Latin for shield), was the standard, rectangular, semi-cylindrical shield carried by Roman legionaries during the Principate. Republican-era scuta had the form of an oval, and in the late fourth century the Roman Army began to exchange their rectangular scuta for oval or large circular shields. The cingulum was a military belt worn at all times, even without the rest of the armor.
The gladius was the short sword, 18 to 24 inches long, used by Roman legionaries from the 3rd century BC until the late Roman Empire. It was primarily used for stabbing and thrusting. The gladius was made by Spaniards The hasta was a spear used by triarii in the times of the Republic, and also as the primary weapon of the hastati and principes in the early Republic. The pilum, was a specialized javelin that would bend after being thrown to prevent enemies from re-using it. Additionally, in the army of the late empire, the gladius was often replaced by a spatha (longsword), up to 1 meter long, the rectangular scutum was dropped in favor of an oval shield, the earlier pilum had evolved into a differently shaped javelin - lighter and with a greater range - and new weapon types such as thrown darts (plumbatae) were introduced. (Santosuosso, A., Soldiers, Emperors and Civilians in the Roman Empire, Westview, 2001, p. 190)
The scorpio, or "dart thrower," was a large crossbow used by one man. The ballista, was larger than the scorpio, and also derived from the crossbow. The onager, was a siege engine used against fortifications. The catapulta was a machine that hurled javelins.
Senior officers Legatus Legionis/legate: The overall Legionary commander. This post was generally appointed by the emperor, 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 Quaestor: Served as a type of quartermaster general, in charge of purchasing, finance, the collection and distribution of booty, etc. Again, these might perform similar functions on the civilian side. The Legati: senior commanders under the supremo. Generally they were of senatorial rank and were commissioned by the Senate. 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. Praefectus Castrorum: The camp Prefect. Generally he was a long-serving veteran who had been promoted through the ranks of the centurions and was third in overall command. 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. Evocarti: A veteran of the Roman army who has the right to retire, but has chosen to stay on after his tenure has finished. During this period they receive double pay and are often excluded from regular duties such as manual labour.
Roman centurion.Primus Pilus: The senior centurion of the legion and commander of the first cohort was called the primus pilus ("first file", commonly mistaken with primus pilum, and mistranslated as "first spear"), a career soldier and advisor to the legate. While every normal cohort was composed of 5 to 8 centuries, the one that was led by the primus pilus (the first) had about 10 centuries, or 800 men. Pilus Prior: Senior centurion in any cohort other than the first cohort. Commanded that cohort and served as an advisor to the legion's commander. Centurions: They were career soldiers who formed "the backbone of the professional army." They were responsible for the day to day life of the soldiers and were the field commanders. While generally promoted from the ranks, in some cases they could be appointed by the Emperor or other senior officials. There were 64 centurions in each legion (10 in the first cohort and 6 in the rest), one to command each centuria of the 10 cohorts. The ranking of centurions were: Pilus Prior, Pilus Posterior, Princepes Prior, Princepes Posterior, Hastatus Prior, and Hastatus Posterior. The Prior centurion of each pair commanded the maniple. 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. The next step up would be a post as a centurion. Optio: One for each centurion (therefore, there were 64 in a legion), they were appointed by the centurion from within the ranks to act as his second in command. Tesserarius (Guard Commander): Again there were 64 of these, or one for each centuria. They acted in similar roles to the optiones. Signifer: Each centuria had a signifer (therefore, there were 64 in a legion). 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 centuria would rally around. A soldier could also gain the position of Discentes signiferorum, or standard bearer in training. 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 troops' loyalty to him.
Rank and file Legionarii: The basic soldiers of a Legion, who were well trained fighters and received roughly the same pay as Immunes. Immunes: These were trained specialists, such as surgeons, engineers, surveyors, and architects, as well as craftsmen. They were exempt from camp and hard labor duties due to the nature of their work, and would generally earn slightly more pay than the Milites. They also did not fight as much as the normal Milites. Discentes: Milites in training for an immunis position. Milites Gregarii: The basic private-level foot soldiers. Tirones: The basic new private recruits. A tiro could take up to 6 months before becoming a full miles.
Fitness The main prerequisite for a member of the Roman Army was fitness. The first thing the soldiers were taught to do was to march. During the summer the soldiers had to marched 18.4 miles in five hours. A further part of basic military training was physical exercise such as long distance running, high and long jumping, climbing over walls and carrying heavy packs with full armour on. During the summer, swimming was also a part of training. If their camp was near the sea, a lake or a river, every recruit was made to swim.They also had to be fit to be able to fight well and cope with any injuries. Stamina was essential. Soldiers would be expected to be able to endure extended periods without food in case of a break in supplies.
Group training Every day the whole of the legion would practice running, jumping, fencing and javelin throwing. But, before that happened, newcomers would do two sessions of military drill and give their oath of loyalty to their commander and Emperor.
Drill and weapons training Both the legionary and auxilia troops also did drill training, from fundamentals such as learning military step and the exact formation of ranks, to practicing tactical maneuvers. Roman tactics also required the soldier to be able to respond instantly to commands to change the shape of his formation, and not simply to fight as a brave individual, as in barbarian armies. This required extensive training and discipline. Weapons training covered how to handle a sword, both to become accustomed to the weight and balance, and also how to deliver blows to an enemy without exposing the soldier's own body to enemy strikes. In contrast to other contemporary styles, Roman sword-fighting was fairly restrained and measured - primarily, the sword was to be used to make short stabbing strokes from behind the protection of the scutum (shield) with minimal risk of the soldier receiving counter-strokes. This is in contrast to the rather looser style of slashing blows favored by many barbarian peoples. A favored tactic was to knock one's opponents off their feet with a ram of the scutum (shield), and then to dispatch him with one or more swift downward stabs while he was vulnerable on the ground, all the while remaining protected by the scutum, which was to remained raised. They also trained in the use of the thrown javelin and pilum.
Common skills At a minimum, it was expected that all troops would be competent at swimming, so they would be able to ford any rivers without the aid of a bridge. Some of them would sometimes have to swim in their armour so they could continue fighting on the other side of the river.
History of the Roman army Main articles: Campaign history of the Roman military, Structural history of the Roman military, Technological history of the Roman military, Political history of the Roman military, Roman legion, and List of Roman legions From a few score men defending a small hill town in Italy, through a citizen militia consisting of citizen-farmers raised annually for a short campaign before returning to harvest their fields, the Roman army grew to be a professional standing army of several hundred thousand men. Roman historian Edward Gibbon estimates in his book The Decline and Fall of the Roman Empire that the peak size of the Roman army in the late imperial period was on the order of 375,000 men.