The Roman Army () served ancient Rome and the Roman people enduring through the Roman Kingdom (753–509 BC), the Roman Republic (509–27 BC), and the Roman Empire (27 BC – AD 476/1453), including the Western Roman Empire (collapsed AD 476/480) and the Eastern Roman Empire (collapsed AD 1453). It is thus a term that broadly spans approximately 2,205 years, during which the force underwent numerous permutations in size, composition, organization, equipment and tactics, while conserving a core of lasting traditions.The Complete Roman Army, Adrian Goldsworthy Thames & Hudson, 2011Companion to the Roman Army, Paul Erdkamp, John Wiley & Sons, 31 March 2011
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 (to c. 500 BC), the standard levy was probably of 9,000 men, consisting of 6,000 heavily armed infantry (probably Greek-style hoplites), plus 2,400 light-armed infantry (rorarii, later called velites) 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 (the Foedus Cassianum), 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 (the phalanx) as in the Early Roman army, the Romans now drew up in three lines consisting of small units (maniples) 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 (218–201 BC) 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 (although formations could be kept in being over winter during major wars). The standard levy was doubled during the Samnite Wars to four legions (two per consul), 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 (age 16–46). The army's senior officers, including its commanders-in-chief, the Roman consuls, were all elected annually at the People's Assembly. Only equites (members of the Roman knightly order) were eligible to serve as senior officers. of the highest social classes ( and the First Class of commoners) provided the legion's cavalry, the other classes the legionary infantry. The proletarii (those assessed at under 400 wealth) 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 (bronze cuirasses and, later, chain-mail shirts). 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 (both Gallic and Greek) 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. (1) The struggle for hegemony over Italy, especially against the Samnite League (338–264 BC); (2) the struggle with Carthage for hegemony in the western Mediterranean Sea (264–201 BC); and (3) the struggle against the Hellenistic monarchies for control of the eastern Mediterranean (201–91 BC). During the earlier phase, the normal size of the levy (including allies) was in the region of 40,000 men (two consular armies of c. 20,000 men each).
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 , a process probably complete by the time the general Marius assumed command in 107 BC. (The so-called "Marian reforms" of the army hypothesised by some scholars are today seen by other scholars as having evolved earlier and more gradually.)
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 and the Social War (91-88 BC). The result was the grant of Roman citizenship to all Italians and the end of the Polybian army's dual structure: the were abolished and the recruited into the legions.
Imperial Roman army (30 BC – AD 284)
Under the founder–emperor Augustus (ruled 30 BC – 14 AD), 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 (conscription was only decreed in emergencies). In the later 1st century, the size of a legion's First Cohort was doubled, increasing legionary personnel to c. 5,500.
Alongside the legions, Augustus established the auxilia, a regular corps of similar numbers to the legions, recruited from the peregrini (non-citizen inhabitants of the empire – about 90% of the empire's population in the 1st century). 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 (heavy and light), light infantry, archers and other specialists. The auxilia were organised in c. 500-strong units called (all-infantry), (all-cavalry) and (infantry with a cavalry contingent attached). 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 (called numeri) 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.