It is thus a term that broadly spans approximately 1229/2,206 years, during which the force underwent numerous permutations in size, composition, organisation, equipment and tactics, while conserving a core of lasting traditions.

[1][2][3] Until c. 550 BC, 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.

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.

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.

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 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.

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.

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).

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.

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.

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.

Alongside the regular forces, the army of the Principate employed allied native units (called numeri) from outside the empire on a mercenary basis.

As all-citizen formations, and symbolic guarantors of the dominance of the Italian hegemony,[citation needed] legions enjoyed greater social prestige than the auxilia.

There was no army general staff in Rome, but the leading praefectus praetorio (commander of the Praetorian Guard) often acted as the emperor's de facto military chief-of-staff.

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.

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 (Roman colonies), and also engaged in large-scale forest clearance and marsh drainage to expand the province's available arable land.

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.

With the benefit of archaeological discoveries of recent decades, many contemporary historians view the late army as no larger than its predecessor: under Diocletian c. 390,000 (the same as under Hadrian almost two centuries earlier) and under Constantine no greater, and probably somewhat smaller, than the Principate peak of c. 440,000.

The interpretation of this trend has fuelled an ongoing debate whether the army adopted a defence-in-depth strategy or continued the same posture of "forward defence" as in the early Principate.

This may have been due to heavier barbarian pressure, and/or to the practice of keeping large armies of the best troops in the interior, depriving the border forces of sufficient support.

At Manzikert and later at Dyrrhachium, units tracing their lineage for centuries back to Late Roman army were wiped out, and the subsequent loss of Asia Minor deprived the Empire of its main recruiting ground.

[4] Yet, through a combination of skill, determination and years of campaigning, Alexios, John and Manuel Komnenos managed to restore the power of the Byzantine Empire by constructing a new army from scratch.

Alongside troops raised and paid for directly by the state the Komnenian army included the armed followers of members of the wider imperial family and its extensive connections.

After Andronikos II took to the throne, the army fell apart and the Byzantines suffered regular defeats at the hands of their eastern opponents, although they would continue to enjoy success against the crusader territories in Greece.

By c. 1350, following a destructive civil war and the outbreak of the Black Death, the Empire was no longer capable of raising troops and the supplies to maintain them.

The Empire came to rely upon troops provided by Serbs, Bulgarians, Venetians, Latins, Genoans and Ottoman Turks to fight the civil wars that lasted for the greater part of the 14th century, with the latter foe being the most successful in establishing a foothold in Thrace.

Readers seeking presentations of the Roman army by theme, rather than by chronological phase, should consult these articles: History Corps Strategy and tactics Equipment Other