Until ca. 550 BC, there was probably 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. During the Regal Era (to ca. 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 (equites celeres). When the kings were replaced by two annually-elected praetores in ca. 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. 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 (CA. 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 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 ca. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by an non-citizen formation of roughly equal size, the ala, recruited from Rome's Italian allies, or socii. 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 4 legions (2 per Consul), for a total of ca. 18,000 Roman troops and 4 allied alae of similar size. For the vast majority of the period of its existence, the Polybian levy was at war. This led to great strains on Roman and Italian manpower, but forged a superb fighting machine. During the Second Punic War, fully two-thirds of Roman iuniores were under arms continuously. 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 (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 alae were abolished and the socii recruited into the legions.
IMPERIAL ROMAN ARMY (30 BC - AD 284)
Under the founder–emperor Augustus (ruled 30 BC – 14 AD), the legions, ca. 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 ca. 5,500. By the end of Augustus' reign, the imperial army numbered some 250,000 men, equally split between legionaries and auxiliaries 25 legions and ca. 250 auxiliary regiments). The numbers grew to a peak of about 450,000 by 211 (33 legions and ca. 400 auxiliary regiments). 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 ca. 400,000 (but probably not to their 211 peak) under Diocletian (r. 284-305). After the empire's borders became settled (on the Rhine-Danube line in Europe) by 68, virtually all military units (except the Praetorian Guard) were stationed on or near the borders, in roughly 17 of the 42 provinces of the empire in the reign of Hadrian (r. 117-38). Soldiers, mostly drawn from polytheistic societies, enjoyed wide freedom of worship in the polytheistic Roman system. They revered both 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.
LATE ROMAN ARMY/EAST ROMAN ARMY (284 – 641)
The size of the 4th century army is controversial. More dated scholars (e.g. A.H.M. Jones, writing in the 1960s) estimated the late army as much larger than the Principate army, half the size again or even as much as twice the size. With the benefit of archaeological discoveries of recent decades, many contemporary historians view the late army as no larger than its predecessor: under Diocletian ca. 390,000 (the same as under Hadrian almost 2 centuries earlier) and under Constantine no greater, and probably somewhat smaller, than the Principate peak of ca. 440,000. The main change in structure was the establishment of large armies that accompanied the emperors (comitatus praesentales) and were generally based away from the frontiers. Their primary function was to deter usurpations. The legions were split up into smaller units comparable in size to the auxiliary regiments of the Principate. In parallel, legionary armour and equipment were abandoned in favour of auxiliary equipment. Infantry adopted the more protective equipment of the Principate cavalry.
BYZANTINE ARMY (641-1071)
Less than thirty years after the death of emperor Justinian, when the emperor Tiberius II Constantinus succeeded to the throne in AD 578, the army was further reorganized. One of the emperor's leading generals, who was later to become emperor himself, Maurice, issued the strategicon, a handbook of the workings of the army of the eastern empire. The Byzantine army possessed not only the Roman traditions of strategy but also a complete system of tactics suited to the conflicts of the age. Greek expressions, as well as some Germanic terms, are now in some cases beginning to take the place of the former Latin ones. Though Latin still remained the language of the army. The forces were now organized in numeri, an expression for some units which appeared to have come into use as early as Diocletian or Constantine. The numeri, or war-bands (bandae), were not necessarily all of the same size. In fact the Byzantine army appeared to take great care not to have all its units of the same size, in order to confuse an opponent in battle as to where its strengths and weaknessses lay. (A system still used by Napoleon.) A numerus which was between three or four hundred men strong and was commanded by a comes or tribunus. If several numeri could form a brigade (drungus) of two to three thousand men, which would be commanded by a dux. These brigades again could unite to form a division (turma) of up six to eight thousand men. During peacetime these forces were not united into brigades and divisions, far more they were spread across the territories. It was only at the outbreak of war that the commander would weld them into a force. Also part of the reorganization was the end of the comitatus system by which the soldiers owed their loyalty to their commander. Now the soldiers' loyalties lay with the emperor. This change was made easy by the fact that the German federates who had brought in such customs were now in the decline within the eastern army. As the amount of money available to the government declined so too did the number of German mercenaries decrease. The remaining German mercenaries were to be found divided into foederati (federates), optimati (the best men picked from the federates), buccellarii (the emperor's bodyguard). The optimati are of particular interest as they appear clearly to resemble the fore-runners of medieval knights. They were chosen bands of German volunteers, who appeared to be of such standing among their own people that they each brought with them one or two armati, who were their personal assistants, just as later squires attendend to their knights.
KOMNENIAN BYZANTINE ARMY (1071 – 1204)
The Komnenian period marked a rebirth of the Byzantine army. At the beginning of the Komnenian period in 1081, the Byzantine Empire had been reduced to the smallest territorial extent in its history. Surrounded by enemies, and financially ruined by a long period of civil war, the empire's prospects had looked grim. The state lay defenseless before, as the Byzantine army had been reduced to a shadow of its former self: during the 11th century, decades of peace and neglect had reduced the old thematic forces, and the Battle of Manzikert in 1071 had destroyed the professional tagmata, the core of the Byzantine army. At Manzikert, units tracing their lineage for centuries back to the Roman Empire were wiped out, and the subsequent loss of Asia Minor deprived the Empire of its main recruiting ground. In the Balkans, at the same time, the Empire was exposed to invasions by the Norman Kingdom of Sicily, and by Pecheneg raids across the Danube. In 1097, the Byzantine army numbered around 70,000 men altogether. By 1180 and the death of Manuel Komnenos, whose frequent campaigns had been on a grand scale, the army was probably considerably larger. During the reign of Alexios I, the field army numbered around 20,000 men which was increased to about 30,000 men in John II's reign. By the end of Manuel I's reign the Byzantine field army had risen to 40,000 men.
PALAIOLOGAN BYZANTINE ARMY (1261-1453)
The Palaiologan army refers to the military forces of the Byzantine Empire from the late 13th century to its final collapse in the mid 15th century, under the House of the Palaiologoi. The army was a direct continuation of the forces of the Nicaean army, which itself was a fractured component of the formidable Komnenian army. Under the first Palaiologan emperor, Michael VIII, the army's role took an increasingly offensive role whilst the naval forces of the Empire, weakened since the days of Andronikos I Komnenus was boosted to included thousands of skilled sailors and some 80 ships. Due to the lack of land to support the army, the Empire required the use of large numbers of mercenaries.
The infantry (armatura) was heavy, because they had helmets (cassis), coats of mail (catafracta), greaves (ocrea), shields (scutum), larger swords (gladius maior), which they call broadswords (spatha), and some smaller, which they name half-broadswords (semispathium), five weighted darts (plumbata) placed in the shields, which they hurl at the beginning of the assault, then double throwables, a larger one with an iron point of nine ounces and a stock of five and one-half feet, which was called a pilum, but now is called a spiculum, in the use of which the soldiers were especially practiced, and with skill and courage could penetrate the shields of the infantry and the mail of the cavalry. The other smaller had five ounces of iron and a stock of three and one-half feet, and was called a vericulum but now is a verutum. The first line, of hastati, and the second, of principes, were composed of such arms. Behind them were the bearers (ferentarius) and the light infantry, whom now we say are the supporters and the infantry, shield-bearers (scutum) with darts (plumbata), swords (gladius) and , armed just as are nearly all soldiers today. There were likewise bowmen (sagittarius) with helmet (cassis), coat of mail (catafracta), sword (gladius), arrows (sagitta) and bow (arcus). There were slingers (funditor) who slung stones (lapis) in slings (funda) or cudgel-throwers (fustibalus). There were artillery-men (tragularius), who shot arrows from the manuballista and the arcuballista.