The Byzantine army was the primary military body of the Byzantine armed forces, serving alongside the Byzantine navy. A direct continuation of the Eastern Roman army, shaping and developing itself on the legacy of the late Hellenistic armies, it maintained a similar level of discipline, strategic prowess and organization. It was among the most effective armies of western Eurasia for much of the Middle Ages. Over time the cavalry arm became more prominent in the Byzantine army as the legion system disappeared in the early 7th century. Later reforms reflected some Germanic and Asian influences—rival forces frequently became sources of mercenary units, such as the Huns, Cumans, Alans and (following the Battle of Manzikert) Turks, meeting the Empire's demand for light cavalry mercenaries. Since much of the Byzantine military focused on the strategy and skill of generals utilizing militia troops, heavy infantry were recruited from Frankish and later Varangian mercenaries.
From the 6th to the 12th centuries, the Byzantine army was among the most powerful and effective military forces in the world – neither Middle Ages Europe nor (following its early successes) the fracturing Caliphate could match the strategies and the efficiency of the Byzantine army. Restricted to a largely defensive role in the 7th to mid-9th centuries, the Byzantines developed the theme system to counter the more powerful Caliphate. From the mid-9th century, however, they gradually went on the offensive, culminating in the great conquests of the 10th century under a series of soldier-emperors such as Nikephoros II Phokas, John Tzimiskes and Basil II. The army they led was less reliant on the militia of the themes; it was by now a largely professional force, with a strong and well-drilled infantry at its core and augmented by a revived heavy cavalry arm. With one of the most powerful economies in the world at the time, the Empire had the resources to put to the field a powerful host when needed, in order to reclaim its long-lost territories.
After the collapse of the theme system in the 11th century, the Byzantines grew increasingly reliant on professional Tagmata troops, including ever-increasing numbers of foreign mercenaries. The Komnenian emperors made great efforts to re-establish a native army, instituting the pronoia system of land grants in exchange for military service. Nevertheless, mercenaries remained a staple feature of late Byzantine armies since the loss of Asia Minor reduced the Empire's recruiting-ground, while the abuse of the pronoia grants led to a progressive feudalism in the Empire. The Komnenian successes were undone by the subsequent Angeloi dynasty, leading to the dissolution of the Empire at the hands of the Fourth Crusade in 1204.
The Emperors of Nicaea managed to form a small but effective force using the same structure of light and heavily armed troops, both natives and foreigners. It proved effective in defending what remained of Byzantine Anatolia and reclaiming much of the Balkans and even Constantinople itself in 1261. Another period of neglect of the military followed in the reign of Andronikos II Palaiologos, which allowed Anatolia to fall prey to an emerging power, the Ottoman Empire. Successive civil wars in the 14th century further sapped the Empire's strength and destroyed any remaining chance of recovery, while the weakening of central authority and the devolution of power to provincial leaders meant that the Byzantine army was now composed of a collection of militias, personal entourages and mercenary detachments.
History In the period after the Muslim conquests, which saw the loss of Syria and Egypt, the remnants of the provincial armies were withdrawn and settled in Asia Minor, initiating the thematic system. Despite this unprecedented disaster, the internal structures of the army remained much the same, and there is a remarkable continuity in tactics and doctrine between the 6th and 11th centuries. The Battle of Manzikert in 1071 and the subsequent Seljuk invasions, together with the arrival of the Crusades and the incursions of the Normans, would severely weaken the Byzantine state and its military, which increasingly had to rely on foreign mercenaries.
The army under Diocletian and Constantine
The Eastern Empire dates from the creation of the Tetrarchy ("Quadrumvirate") by the Emperor Diocletian in 293. His plans for succession did not outlive his lifetime, but his reorganization of the army did by centuries. Rather than maintain the traditional infantry-heavy legions, Diocletian reformed it into ("border") and (field armies). However, the last legion, Legio V Macedonica, survived until 637, when it was destroyed during the Arab invasion of Egypt.Ross Cowan, The Longest Lived Legion, Ancient Warfare
There was an expansion of the importance of the cavalry, though the infantry still remained the major component of the Roman armies, in contrast to common belief. In preparation for Justinian's African campaign of 533–534 AD, the army assembled amounted to 10,000-foot soldiers and 5,000 mounted archers and federate lancers.
The and were to occupy the limes, the Roman border fortifications. The field units, by contrast, were to stay well behind the border and move quickly where they were needed, whether for offensive or defensive roles, as well as forming an army against usurpers. The field units were held to high standards and took precedence over Limitanei in pay and provisions.
Cavalry formed about one-third of the units, but as a result of smaller units, about one-quarter of the Roman armies consisted of cavalry. About half the cavalry consisted of heavy cavalry (including the ). They were armed with spear or lance and sword and armored in mail. Some had bows, but they were meant for supporting the charge instead of independent skirmishing.
In the field armies there was a component of some 15% of cataphractarii or clibanarii, heavily armoured cavalry who used shock tactics. The light cavalry (including the and ) featured high amongst the limitanei, being very useful troops on patrol. They included horse archers (). The infantry of the comitatenses was organized in regiments (variously named , or just ) of about 500–1,200 men. They were still the heavy infantry of old, with a spear or sword, shield, body armour and a helmet. But now each regiment was supported by a detachment of light infantry skirmishers.
If needed, the infantry could take off (some of) their armour to act in a more flexible way as Modares did (according to Zosimus) during the Gothic War of the 370s. The regiments were commanded by a ("tribune") and brigaded in pairs (cavalry units did, too) under a comes. These brigades probably were tactical and strategic units only, as no traces survive of brigade staff corps.
On the other hand, little is known of the limitanei. The old legions, cohorts and cavalry alae survived there, and newer units were created (the new legions, or and ), amongst the cavalry. The limitanei infantry may have been lighter-equipped than the comitatenses infantry, but there is no evidence whatsoever. They were paid less than the field troops and recruited locally. Consequently, they were of inferior quality. However, they were in the line of fire. They countered most incursions and raids. Thus, it can be assumed they had superior field experience (except in periods of long campaigning for the comitatenses), though that experience did not extend to large battles and sieges.
The Scholae Palatinae units, which were more properly known as the and the "Protective Association of the Royal Escort" (also called the ), were the personal guard of the Emperor, and were created to replace the Praetorian Guard disbanded by Constantine I.
Following a major reorganisation of the Roman army during the Emperor Diocletian's reign (284–305 AD) the legions in the third and fourth century bore little resemblance to those of the Republic or earlier Roman empire. Reduced in numbers to about 1,000 men per legion, these units became static garrison troops, sometimes serving on a part-time militia basis as hereditary . As such they were separate from the new mobile field army.
The army of Justinian I and his successors
The army of Justinian I was the result of fifth-century reorganizations to meet growing threats to the empire, the most serious from the expanding Persian empire. Gone were the familiar legions, cohorts and alae of old Rome, and in their place were small Greek infantry battalions or horse regiments called an arithmos, tagma or numerus. A numerus had between 300 and 400 men and was commanded by a Stratarches. Two or more numeri formed a brigade, or moira; two or more brigades a division, or meros.
There were six classifications of troops:
The guard troops stationed in the capital. The comitatenses of the old Roman field armies. In Justinian's day these were more commonly called stratiotai. Regular soldiers of the early Byzantine army, the stratiotai were chiefly recruited from subjects of the empire in the highlands of Thrace, Illyricum and Isauria. The limitanei. The least changed element of the Roman army, limitanei still performed their traditional duties of guarding frontiers and garrisoning border posts. Like how the comitatenses were called stratiotai in the heyday of the Justinians, the limitanei were known as akritai by the mainly Greek-speaking subjects of the Eastern Empire. This terming of limitanei as akritai in Greek, led to folktales of the heroism of the limitanei/akritai, especially the popular tale of the hero Digenes Akritas during the wars between the Byzantines and the various Arab Caliphates. The foederati. They were a relatively new element in the army, recruited from the fifth century onwards from barbarian volunteers. They were formed into cavalry units under Byzantine "Gothic" officers. A ban on enlistments by Byzantine subjects was lifted in the sixth century, and their composition became mixed. The Allies. These were bands of barbarians, Huns, Herules, Goths or others who were bound by treaty to provide the empire with military units commanded by their own chiefs, in return for land or yearly subsidy. The bucellarii. The private armed retainers of generals, Praetorian Prefects, officers of lesser rank and the rich, the bucellarii were often a significant portion of a field army's cavalry force. The size of a retinue of bucellarii depended on the wealth of the employer. Their rank and file were called , or shield-bearers, and their officers, or spear-bearers. took solemn oaths of fidelity to their patron and of loyalty to the emperor. One of the most noted generals of the period, Belisarius, had been a in Justinian's retinue before his becoming emperor. The bucellarii were usually mounted troops, mostly Huns, Goths and mountaineers of Thrace or Asia Minor.
The size of Justinian's army is unclear. Bury, writing in the 1920s, accepted the estimate of 150,000 troops of all classes in 559 given by Agathia of Myrina in his History. Norwich also estimated the army under Justinian at 150,000 soldiers. Other scholars estimate the total strength of the imperial army under Justinian to be between 300,000 and 350,000 soldiers.Haldon, Warfare, State And Society In The Byzantine World, 565-1204, 1999, p. 100Bernheimer, Silverstein, Late Antiquity: Eastern Perspectives, 2012Crone, The Qurʾānic Pagans and Related Matters, 2016, p. 25Parnell, David Alan. Justinian's Men: Careers and Relationships of Byzantine Army Officers, 518 - 610, 2016, p.19-20 Field armies generally had 15,000 to 25,000 soldiers and were formed mainly of comitatenses and foederati, reinforced by the commanders' retinues and barbarian allies. The expeditionary force of Belisarius during his reconquest of Carthage from the Vandals in 533 is illustrative.