Imperial Roman army
The Imperial Roman Army was the military land force of the Roman Empire from 27 BC to 476 AD, and the final incarnation in the long history of the Roman army. This period is sometimes split into the Principate and the Dominate periods.
Under Augustus, the army consisted of legions, eventually auxilia and also numeri. By the end of Augustus' reign, the imperial army numbered some 250,000 men, equally split between 25 legions and 250 units of auxiliaries. The numbers grew to a peak of about 450,000 by 211, in 33 legions and about 400 auxiliary units. By then, auxiliaries outnumbered legionaries substantially. From this peak, numbers probably underwent a steep decline by 270 due to plague and losses during multiple major invasions by the Germanic Tribal Folk. Numbers were restored to their early 2nd-century level of c. 400,000 under Diocletian.
After the Empire's borders became settled by AD 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.
History
Background
The army of the late Republic that Augustus took over on becoming sole ruler of the Empire in 27 BC consisted of a number of large formations called legions, which were composed exclusively of heavy infantry. The legion's light infantry which had been deployed in earlier times, had been phased out as had its contingent of cavalry. Legions were recruited from Roman citizens only, by regular conscription, although by 88 BC, a substantial proportion of recruits were volunteers.To remedy the deficiencies in capability of the legions, the Romans relied on a motley array of irregular units of allied troops, both composed of subject natives of the empire's provinces and of bands supplied, often on a mercenary basis, by Rome's allied kings beyond the Empire's borders. Led by their own aristocrats and equipped in their own traditional fashion, these native units varied widely in size, quality and reliability. Most would only be available for particular campaigns before returning home or disbanding.
Establishment
On gaining undisputed mastery over the Roman empire in 27 BC, Augustus was left with an army which was bloated by extraordinary recruitment for the Roman civil wars and at the same time lacking a suitable organisation for the defence and expansion of a vast empire. Even after disbanding most of his defeated adversary Mark Antony's legions, Augustus had 50 legions under his command, composed exclusively of Roman citizens i.e., by that time, of Italians and inhabitants of Roman colonies outside Italy. Alongside these were a mass of irregular non-Italian allied units whose command, size and equipment varied greatly. Some allied units came from provinces within the empire, others from beyond the imperial borders.Legions
The first priority was to reduce the number of legions to a sustainable level. Fifty legions implied too high a recruitment burden for a male citizen-body only about two-million strong, especially as Augustus intended to create a long-term career force. The Emperor retained just over half his legions, disbanding the rest and settling their veterans in no less than 28 new Roman colonies. The number of legions remained close to that level throughout the Principate.Unlike the Republican legions, which were, in theory at least, temporary citizen-levies for the duration of particular wars, Augustus and his right-hand man Agrippa clearly envisioned their legions as permanent units composed of career professionals. Under the late Republic, a Roman citizen iunior could legally be required to serve a maximum of sixteen years in the legions and a maximum of six years consecutively. The average number of years served was about ten. In 13 BC, Augustus decreed sixteen years as the standard term of service for legionary recruits, with a further four years as reservists. In AD 5, the standard term was increased to twenty years plus five years in the reserves. In the period following its introduction, the new term was deeply unpopular with the troops. On Augustus' death in AD 14, the legions stationed on the rivers Rhine and Danube staged major mutinies, and demanded, among other things, reinstatement of a sixteen-year term. Augustus prohibited serving legionaries from marrying, a decree that remained in force for two centuries. This measure was probably prudent in the early imperial period, when most legionaries were from Italy or the Roman colonies on the Mediterranean, and were required to serve long years far from home. This could lead to disaffection if they left families behind. But from about AD 100 onwards, when most legions were based long-term in the same frontier-province and recruitment was primarily local, the prohibition of marriage became a legal encumbrance that was largely ignored. Many legionaries formed stable relationships and brought up families. Their sons, although illegitimate in Roman law and thus unable to inherit their fathers' citizenship, were nevertheless frequently admitted to legions.
At the same time, the traditional grant of land to retiring veterans was made replaceable by a cash discharge bonus, as there was no longer sufficient state-owned land in Italy to distribute. Unlike the Republic, which had relied primarily on conscription, Augustus and Agrippa preferred volunteers for their professional legions. Given the onerous new term of service, it was necessary to offer a substantial bonus to attract sufficient citizen-recruits. In AD 5, the discharge bonus was set at 3,000 denarii. This was a generous sum equivalent to about 13 years' gross salary for a legionary of the time. To finance this major outlay, Augustus decreed a 5% tax on inheritances and 1% on auction-sales, to be paid into a dedicated aerarium militare. However, veterans continued to be offered land instead of cash in Roman colonies established in the newly annexed frontier provinces, where public land was plentiful. This was another grievance behind the mutinies of 14 AD, as it effectively forced Italian veterans to settle far from their own country. The imperial authorities could not compromise on this issue, as the planting of colonies of Roman veterans was a crucial mechanism for controlling and Romanising a new province, and the foundation of veterans' colonies did not cease until the end of Trajan's rule. But as legionary recruitment became more localised, the issue became less relevant.
File:Roman Legionaries-MGR Lyon-IMG 1050.JPG|thumb|left|Imperial Roman legionaries in testudo formation, a relief from Glanum, a Roman town in what is now southern France that was inhabited from 27 BC to 260 AD
Augustus modified the command structure of the legion to reflect its new permanent, professional nature. In Republican tradition, each legion was under six equestrian military tribunes who took turns to command it in pairs. But in the late Republic, military tribunes were eclipsed by higher-ranking officers of senatorial rank called legati. A proconsul might ask the senate to appoint a number of legati to serve under him e.g., Julius Caesar, Augustus' grand-uncle and adoptive father, had five, and later ten, legati attached to his staff when he was governor of Cisalpine Gaul. These commanded detachments of one or more legions at the governor's behest and played a critical role in the conquest of Gaul. But legions still lacked a single, permanent commander. This was provided by Augustus, who appointed a legatus to command each legion with a term of office of several years. The ranking senatorial military tribune was designated deputy commander, while the remaining five equestrian tribunes served as the legatus' staff officers. In addition, Augustus established a new post of praefectus castrorum, to be filled by a Roman knight. Technically, this officer ranked below the senatorial tribune, but his long operational experience made him the legion commander's de facto executive officer. The prefect's primary role was as the legion's quartermaster, in charge of legionary camps and supplies.
It has been suggested that Augustus was responsible for establishing the small cavalry contingent of 120 horse attached to each legion. The existence of this unit is attested in Josephus' Bellum Iudaicum written after AD 70, and on a number of tombstones. The attribution to Augustus is based on the assumption that legionary cavalry had completely disappeared in the Caesarian army. The Augustan era also saw the introduction of some items of more sophisticated and protective equipment for legionaries, primarily to improve their survival rate. The lorica segmentata, was a special laminated-strip body-armour, was probably developed under Augustus. Its earliest depiction is on the Arch of Augustus at Susa, dating from 6 BC. The oval shield of the Republic was replaced by the convex rectangular shield of the imperial era.
Auxilia
Augustus' ambitious expansion plans for the Empire soon proved that 28 legions were not sufficient. Starting with the Cantabrian Wars, which aimed to annex the mineral-rich mountains of north-western Spain, Augustus' 41-year sole rule saw an almost uninterrupted series of major wars that frequently stretched the army's manpower to the limit.Augustus retained the services of numerous units of irregular allied native troops. But there was an urgent need for extra regular troops, organised, if not yet equipped, in the same way as the legions. These could only be drawn from the Empire's vast pool of non-citizen subjects, known as peregrini. These outnumbered Roman citizens by around nine to one in the early 1st century. The peregrini were now recruited into regular units of cohort-strength, to form a non-citizen corps called the auxilia. By AD 23, Tacitus reports that the auxilia numbered roughly as many as the legionaries. The roughly 250 regiments of auxilia this implies were divided into three types: an all-infantry cohors ; an infantry unit with a cavalry contingent attached, the cohors equitata ; and an all-cavalry ala, of which c. 50 were originally established.
It appears that at this early stage, auxiliary recruitment was ethnically based, with most men originating from the same tribe or province. Hence regiments carried an ethnic name e.g., cohors V Raetorum, recruited from the Raeti, a group of Alpine tribes that inhabited modern Switzerland. It has been suggested that the equipment of auxiliary regiments was not standardised until after AD 50, and that until then, auxiliaries were armed with the traditional weaponry of their tribe. But it is possible that at least some regiments had standardised equipment from Augustan times.
Auxiliary regiments were designed to operate as a complement to the legions. That is, they performed exactly the same role as the Republic's alae of Italian allies before the Social War, an equal number of which always accompanied legions on campaign.