Roman army
The Roman army served ancient Rome and the Roman people, enduring through the Roman Kingdom, the Roman Republic, and the Roman Empire, including the Western Roman Empire and the Eastern Roman Empire. It is thus a term that broadly spans approximately 2,206 years, during which the force underwent numerous permutations in size, composition, organization, equipment and tactics, while conserving a core of lasting traditions.
Early Roman army (c. 550 – c. 300 BC)
Until, there was no "national" Roman army, but a series of clan-based war-bands which only coalesced into a united force in periods of serious external threat. Around 550 BC, during the period conventionally known as the rule of king Servius Tullius, it appears that a universal levy of eligible adult male citizens was instituted. This development apparently coincided with the introduction of heavy armour for most of the infantry. Although originally low in numbers, the Roman infantry was extremely effective and developed some influential battle strategies.The early Roman army was based on a compulsory levy from adult male citizens which was held at the start of each campaigning season, in those years that war was declared. There were no standing or professional forces. During the regal period, the standard levy was probably of 9,000 men, consisting of 6,000 heavily armed infantry, plus 2,400 light-armed infantry and 600 light cavalry. When the kings were replaced by two annually elected praetores in c. 500 BC, the standard levy remained of the same size, but was now divided equally between the praetors, each commanding one legion of 4,500 men.
It is likely that the hoplite element was deployed in a Greek-style phalanx formation in large set-piece battles. However, these were relatively rare, with most fighting consisting of small-scale border-raids and skirmishing. In these, the Romans would fight in their basic tactical unit, the centuria of 100 men. In addition, separate clan-based forces remained in existence until at least, although they would operate under the Praetors' authority, at least nominally.
In 493 BC, shortly after the establishment of the Roman Republic, Rome concluded a perpetual treaty of military alliance, with the combined other Latin city-states. The treaty, probably motivated by the need for the Latins to deploy a united defence against incursions by neighbouring hill-tribes, provided for each party to provide an equal force for campaigns under unified command. It remained in force until 358 BC.
Roman army of the mid-Republic (c. 300 – 107 BC)
The central feature of the Roman army of the mid-Republic, or the Polybian army, was the manipular organization of its battle-line. Instead of a single, large mass as in the Early Roman army, the Romans now drew up in three lines consisting of small units of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility. This structure was probably introduced in c. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by a non-citizen formation of roughly equal size, the ala, recruited from Rome's Italian allies, or socii. The latter were approximately 150 autonomous states which were bound by a treaty of perpetual military alliance with Rome. Their sole obligation was to supply to the Roman army, on demand, a number of fully equipped troops up to a specified maximum each year.The Second Punic War saw the addition of a third element to the existing dual Roman/Italian structure: non-Italian mercenaries with specialist skills lacking in the legions and alae: Numidian light cavalry, Cretan archers, and Balearic slingers. From this time, these units always accompanied Roman armies.
The Republican army of this period, like its earlier forebear, did not maintain standing or professional military forces, but levied them, by compulsory conscription, as required for each campaigning season and disbanded thereafter. The standard levy was doubled during the Samnite Wars to four legions, for a total of c. 18,000 Roman troops and four allied alae of similar size. Service in the legions was limited to property-owning Roman citizens, normally those known as iuniores. The army's senior officers, including its commanders-in-chief, the Roman consuls, were all elected annually at the People's Assembly. Only equites were eligible to serve as senior officers. Iuniores of the highest social classes provided the legion's cavalry, the other classes the legionary infantry. The proletarii were ineligible for legionary service and were assigned to the fleets as oarsmen. Elders, vagrants, freedmen, slaves and convicts were excluded from the military levy, save in emergencies.
The legionary cavalry also changed, probably around 300 BC onwards from the light, unarmoured horse of the early army to a heavy force with metal armour. Contrary to a long-held view, the cavalry of the mid-Republic was a highly effective force that generally prevailed against strong enemy cavalry forces until it was decisively beaten by the Carthaginian general Hannibal's horsemen during the Second Punic War. This was due to Hannibal's greater operational flexibility owing to his Numidian light cavalry.
The Polybian army's operations during its existence can be divided into three broad phases. The struggle for hegemony over Italy, especially against the Samnite League ; the struggle with Carthage for hegemony in the western Mediterranean Sea ; and the struggle against the Hellenistic monarchies for control of the eastern Mediterranean. During the earlier phase, the normal size of the levy was in the region of 40,000 men.
File:Marius Chiaramonti Inv1488.jpg|thumb|upright|Gaius Marius, to whom later historians would misattribute putative reforms of the Roman army
During the latter phase, with lengthy wars of conquest followed by permanent military occupation of overseas provinces, the character of the army necessarily changed from a temporary force based entirely on short-term conscription to a standing army in which the conscripts, whose service was in this period limited by law to six consecutive years, were complemented by large numbers of volunteers who were willing to serve for much longer periods. Many of the volunteers were drawn from the poorest social class, which until the Second Punic War had been excluded from service in the legions by the minimum property requirement: during that war, extreme manpower needs had forced the army to ignore the requirement, and this practice continued thereafter. Maniples were gradually phased out as the main tactical unit, and replaced by the larger cohorts used in the allied alae, a process probably complete by the time the general Marius assumed command in 107 BC.
In the period after the defeat of Carthage in 201 BC, the army was campaigning exclusively outside Italy, resulting in its men being away from their home plots of land for many years at a stretch. They were assuaged by the large amounts of booty that they shared after victories in the rich eastern theatre. But in Italy, the ever-increasing concentration of public lands in the hands of big landowners, and the consequent displacement of the soldiers' families, led to great unrest and demands for land redistribution. This was successfully achieved, but resulted in the disaffection of Rome's Italian allies, who as non-citizens were excluded from the redistribution. This led to the mass revolt of the socii and the Social War. The result was the grant of Roman citizenship to all Italians and the end of the Polybian army's dual structure: the alae were abolished and the socii recruited into the legions.
Imperial Roman army (30 BC – AD 284)
Under the founder–emperor Augustus, the legions, c. 5,000-strong all-heavy infantry formations recruited from Roman citizens only, were transformed from a mixed conscript and volunteer corps serving an average of 10 years, to all-volunteer units of long-term professionals serving a standard 25-year term. In the later 1st century, the size of a legion's First Cohort was doubled, increasing legionary personnel to c. 5,500.File:Engineering corps traian s column river crossing.jpg|thumb|left|Roman auxiliary infantry crossing a river. They can be distinguished by the oval shield they were equipped with, in contrast to the rectangular scutum carried by legionaries. Panel from Trajan's Column, Rome
Alongside the legions, Augustus established the auxilia, a regular corps of similar numbers to the legions, recruited from the peregrini. As well as comprising large numbers of extra heavy infantry equipped in a similar manner to legionaries, the auxilia provided virtually all the army's cavalry, light infantry, archers and other specialists. The auxilia were organised in c. 500-strong units called cohortes, alae and cohortes equitatae. Around 80 AD, a minority of auxiliary regiments were doubled in size. Until about 68 AD, the auxilia were recruited by a mix of conscription and voluntary enlistment. After that time, the auxilia became largely a volunteer corps, with conscription resorted to only in emergencies. Auxiliaries were required to serve a minimum of 25 years, although many served for longer periods. On completion of their minimum term, auxiliaries were awarded Roman citizenship, which carried important legal, fiscal and social advantages. Alongside the regular forces, the army of the Principate employed allied native units from outside the empire on a mercenary basis. These were led by their own aristocrats and equipped in traditional fashion. Numbers fluctuated according to circumstances and are largely unknown.
As all-citizen formations, and symbolic guarantors of the dominance of the Italian hegemony, legions enjoyed greater social prestige than the auxilia. This was reflected in better pay and benefits. In addition, legionaries were equipped with more expensive and protective armour than auxiliaries. However, in 212, the emperor Caracalla granted Roman citizenship to all the empire's inhabitants. At this point, the distinction between legions and auxilia became moot, the latter becoming all-citizen units also. The change was reflected in the disappearance, during the 3rd century, of legionaries' special equipment, and the progressive break-up of legions into cohort-sized units like the auxilia.
By the end of Augustus' reign, the imperial army numbered some 250,000 men, equally split between legionaries and auxiliaries. The numbers grew to a peak of about 450,000 by 211. By then, auxiliaries outnumbered legionaries substantially. From the peak, numbers probably underwent a steep decline by 270 due to plague and losses during multiple major barbarian invasions. Numbers were restored to their early 2nd-century level of c. 400,000 under Diocletian. After the empire's borders became settled by 68, virtually all military units were stationed on or near the borders, in roughly 17 of the 42 provinces of the empire in the reign of Hadrian.
The military chain of command was relatively uniform across the Empire. In each province, the deployed legions' legati reported to the legatus Augusti pro praetore, who also headed the civil administration. The governor in turn reported directly to the emperor in Rome. There was no army general staff in Rome, but the leading praefectus praetorio often acted as the emperor's de facto military chief-of-staff.
Legionary rankers were relatively well-paid, compared to contemporary common labourers. Compared with their subsistence-level peasant families, they enjoyed considerable disposable income, enhanced by periodic cash bonuses on special occasions such as the accession of a new emperor. In addition, on completion of their term of service, they were given a generous discharge bonus equivalent to 13 years' salary. Auxiliaries were paid much less in the early 1st century, but by 100 AD, the differential had virtually disappeared. Similarly, in the earlier period, auxiliaries appear not to have received cash and discharge bonuses, but probably did so from Hadrian onwards. Junior officers, the equivalent of non-commissioned officers in modern armies, could expect to earn up to twice basic pay. Legionary centurions, the equivalent of mid-level commissioned officers, were organised in an elaborate hierarchy. Usually risen from the ranks, they commanded the legion's tactical sub-units of centuriae and cohorts. They were paid several multiples of basic pay. The most senior centurion, the primus pilus, was elevated to equestrian rank upon completion of his single-year term of office. The senior officers of the army, the legati legionis, tribuni militum and the praefecti were all of at least equestrian rank. In the 1st and early 2nd centuries, they were mainly Italian aristocrats performing the military component of their cursus honorum. Later, provincial career officers became predominant. Senior officers were paid very high salaries, multiples of at least 50 times basic.
A typical Roman army during this period consisted of five to six legions. One legion was made up of ten cohorts. The first cohort had five centuria each of 160 soldiers. In the second through tenth cohorts there were six centuria of 80 men each. These do not include archers, cavalry or officers.
File:Colonne trajane 1-43.jpg|thumb|left|Relief scene of Roman legionaries building a road, from Trajan's Column, Museum of Roman Civilization, Rome
Soldiers spent only a fraction of their lives on campaign. Most of their time was spent on routine military duties such as training, patrolling, and maintenance of equipment, etc. Soldiers also played an important role outside the military sphere. They performed the function of a provincial governor's police force. As a large, disciplined and skilled force of fit men, they played a crucial role in the construction of a province's Roman military and civil infrastructure: in addition to constructing forts and fortified defences such as Hadrian's Wall, they built roads, bridges, ports, public buildings, entire new cities, and also engaged in large-scale forest clearance and marsh drainage to expand the province's available arable land.
Soldiers, mostly drawn from polytheistic societies, enjoyed wide freedom of worship in the polytheistic Roman system. They revered their own native deities, Roman deities and the local deities of the provinces in which they served. Only a few religions were banned by the Roman authorities, as being incompatible with the official Roman religion and/or politically subversive, notably Druidism and Christianity. The later Principate saw the rise in popularity among the military of Eastern mystery cults, generally centred on one deity, and involving secret rituals divulged only to initiates. By far the most popular in the army was Mithraism, an apparently syncretist religion which mainly originated in Asia Minor.