Justinian I's reign (527–565) was marked by numerous wars that recovered extensive territories in the western Mediterranean lost by the Roman Empire in the 5th century. As a Christian emperor, Justinian considered it his duty to restore the state's former borders. In the east he continued the war with Persia begun under Justin I, with intermittent conflict lasting until 562. In the west, his campaigns against the barbarian kingdoms succeeded in conquering the Vandal Kingdom in North Africa (533–534) and in bringing Italy under Byzantine control after the war with the Ostrogoths (535–554), while less successful wars with the Visigothic Kingdom expanded Byzantine possessions in Spain. Thanks to the works of Procopius and other Byzantine historians, these wars are well studied. The Byzantine army resembled the earlier Roman army, comprising of the field army (comitatenses) that numbered 150,000, and frontier troops (limitanei), with barbarians playing a significant, though debated, role.
Although Justinian's conquests destroyed several barbarian kingdoms, including the Vandals, Ostrogoths and Visigoths, the empire proved unable to defend itself against new and more dangerous peoples such as the Slavs, Avars and Lombards. Even Constantinople was threatened by attacks from the north in the final years of his reign, and under his successors Justin II, Tiberius II and Maurice the empire increasingly struggled with a shortage of manpower. Researchers attribute this to declining discipline in the "barbarised" army, demographic losses from devastating epidemics and fewer available barbarian mercenaries. By straining resources without accounting for the changed realities of the sixth century, Justinian's campaigns are widely held to have contributed to the subsequent decline of Byzantium, alongside the Plague of Justinian, the final Roman-Persian war, and most significantly, the Arab invasions of the 7th century.
Main directions of foreign policy
In the beginning of Justinian's reign, the empire's neighbors in the west were the so-called "barbarian kingdoms" of the Germans, which had formed in the 5th century on the territory of the Western Roman Empire. In all of these kingdoms the conquerors made up an insignificant minority, and the descendants of the empire's inhabitants, who had inherited Roman culture, could attain high social standing. At the beginning of the 6th century these states flourished under their outstanding rulers — the Franks in northern Gaul under Clovis I, the Burgundians in the Loire valley under Gundobad, the Ostrogoths in Italy under Theodoric the Great, the Visigoths in southern Gaul and Spain under Alaric II, and the Vandals in Africa under Thrasamund. By 527, when Justinian came to the throne, these kingdoms found themselves in a difficult situation. In 508, the Visigoths had been expelled from most of Gaul by the Franks, whose kingdom was later divided among the sons of Clovis. In the first half of the 530s the Burgundians suffered defeat at the hands of the Franks. With the death of Theodoric in 526, a crisis began in the Ostrogothic Kingdom, although even during that ruler's lifetime the conflict between the parties favoring and opposing rapprochement with the Byzantine Empire had already sharpened. A similar situation developed in the early 530s in the Vandal Kingdom.
In the east, Byzantium's only opponent was the Sasanian Empire, with which the empire, with brief interruptions, had waged war since the beginning of the 3rd century. By the beginning of the 6th century, this was a prosperous and developed state, roughly equal in area to Byzantium, stretching from the Indus in the east to Mesopotamia in the west. The main challenges facing the Sasanian state at the beginning of Justinian's reign were the ongoing threat of invasions by the Hephthalite Huns, who had first appeared at the borders in the second half of the 5th century, as well as internal instability and struggles for the shahanshah's throne. Around this time a popular Mazdakism movement arose, directed against the aristocracy and the Zoroastrian priesthood (magi). At the beginning of his reign, Khosrow I (531–579) supported this movement, but by the end of his reign it had come to represent a threat to the state. Under Justin I no significant military events connected with Persia took place. Among the diplomatic events of note is the initiative of Shah Kavad, who in the mid-520-s proposed that Justin adopt his son Khosrow and make him heir to the Roman Empire. This proposal was rejected.
In foreign policy, Justinian's name is associated with the idea of "restoring the Roman Empire," or the "reconquest of the West." The first step in this direction was the conquest of Africa and the subjugation in 533 of the Vandal Kingdom, which had arisen in the territories of Roman North Africa conquered at the beginning of the 5th century. In setting out the goals of this enterprise in his Code of Justinian, the emperor deemed it necessary to seek "vengeance for the wrongs and insults" inflicted by the Arian Vandals on the orthodox church, and "the liberation of the peoples of so great a province from the yoke of slavery." The result of this liberation was to be the opportunity for the population to live "under our happy rule". There are currently two theories concerning when this goal was first formulated. According to one of them, now more widely held, the idea of recovering the West had existed in Byzantium since the end of the 5th century. This view proceeds from the thesis that, after the rise of the barbarian kingdoms professing Arianism, social elements must have persisted that did not accept the loss by Rome of its status as the great city and capital of the civilized world, and did not accept the dominant position of the Arians in the religious sphere. An alternative view, which acknowledges a general desire to return the West to civilisation and orthodox religion, dates the emergence of a programme of concrete action to after the successes in the war with the Vandals. This is supported by various indirect indications, such as the disappearance from legislation and state documents in the first third of the 6th century of words and expressions referring to Africa, Italy and Spain in one way or another, and the loss of Byzantine interest in the empire's first capital.
The well-known Byzantinist George Ostrogorsky saw the origin of Justinian's foreign policy in his religious views. In his opinion, as a Christian ruler, Justinian regarded the Roman Empire as a concept identical to the Christian world, and the victory of the Christian religion was for him as sacred a task as the restoration of Roman power.
Justinian's military affairs
The late Roman army in the 5th–6th centuries
Not much is known about the organization of the armies of the Western and Eastern Roman Empires between 420 and the Anastasian War of 502–506. The army as a whole was divided into units made up of Romans and foederati. In turn, the Roman units were divided into frontier limitanei (Latin: limitanei) and comitatenses (Latin: comitatenses). The limitanei, under the command of duces, guarded their sections of the frontier but could, if necessary, be incorporated into field armies under the command of a magister militum. The magister officiorum presented the emperor with an annual report on the state of the limitanei forces. Service among the limitanei was considered undemanding and was passed down from father to son; they were granted plots of land in return for their service. By the 6th century, the limitanei had lost their effectiveness, and under Justinian they ceased to be counted as part of the army. This decision by Justinian was sharply criticized by Procopius in his Secret History; however, modern researchers interpret it differently, as a shift to paying only for active military service. On the basis of the available evidence, the Byzantinist Michael Whitby concluded that hereditary service among the limitanei had been reasonably attractive.
The comitatenses consisted of two categories of troops: units under the command of the Latin: magister militum praesentalis, stationed in the vicinity of the capital, and provincial armies under the command of the corresponding Latin: magister militum. At the end of the 4th century there were two "presental" armies in Constantinople, three armies in the east (in Illyricum and Thracia), one army in Gaul, a small but important army in Africa, and a number of armies in the West that ceased to exist by 420. The last category of soldiers formed units of foederati (Ancient Greek: φοιδέρατοι). Their independence gradually grew, which played a role in the fall of the empire in the West. Also part of the Byzantine army's structure were units of the imperial guard — the scholae (Latin: scholae), the excubitors (Latin: excubitores), and the candidati (Latin: candidati). By the 6th century the division of the army into limitanei and comitatenses had been preserved. The term "comitatenses" was beginning to fall out of use, and in its place Procopius preferred the words stratiotai (Greek: στρατιώτες) and Ancient Greek: κατάλογοι. In reconquered Africa under Justinian, limitanei were stationed under the earlier terms of land grants.
The comitatenses were subdivided into numeri or arithmoi (Ancient Greek: Άριθμός), which formed the basic military unit of 250–500 soldiers. A larger unit was the moira (moirai) of 2,000–3,000 men, and the meros (meirai) of 6,000–7,000 men. Various theories exist regarding the predominant method of recruiting the army in Justinian's era. According to A. H. M. Jones and John Haldon, the army (at least the field army) consisted predominantly of volunteers who hoped to enrich themselves during campaigns. According to Procopius, soldiers were also entitled to pay that increased with length of service. Other researchers point to the continued existence of compulsory conscription, carried out through landowning taxpayers.
Information on the equipment worn by 6th-century Byzantine soldiers can be found in several surviving military treatises from this period or slightly later (such as the anonymous De Re Strategica and the Strategikon of Maurice), as well as in legal sources. According to these sources, a Byzantine infantryman wore armour and metal thigh guards, beneath which he wore a mail shirt almost 2 centimetres thick. The armour was worn over a thick himation cloak to protect the body from contact with the heavy armour and provide additional protection. They wore a metal helmet with a long spike on top and carried a shield with a diameter of over 1.5 metres for protection. His weapons consisted of a sword, a long, sharp-pointed helmet spike, a bow, and a quiver. Some soldiers carried double-edged axes. The heavy cavalry (cataphracts) was well protected: both rider and horse were clad in iron. The horse's head was fitted with a metal frontlet, and its chest was carefully protected by a cuirass and neck guards; its hooves were shod with iron plates. The cavalryman wore steel armor, carried a shield, and on his head wore a tall helmet adorned with a plume. His weaponry included a sword, a spear, and a quiver. There also existed a less heavily armored light cavalry, but it was of little importance and was used mainly for reconnaissance. Byzantine infantry formations were resistant to the attacks of Persian cavalry. Byzantine army tactics were based on the use of bows by both infantry and cavalry. This approach was effective in combat at a distance, but at close quarters special methods had to be applied to protect the formation.
Various methods were used to recruit the army — conscription and hereditary service for Romans, voluntary enlistment in hope of some benefit, or paid or compulsory service for non-Romans. A centralized system of recruitment took shape under the emperor Zeno (474–491). Under Justinian, a ban on military service remained in place for certain categories of the population: slaves, coloni adscripticii, curiales, and cohortales. However, references to the obligation of sons to inherit their fathers' service, and all indications of conscription, were removed from the legislation. From the few surviving documents concerning the recruitment of limitanei, it is known that this service was hereditary but not compulsory. The abolition of conscription was, in the view of A. H. M. Jones, a revolutionary change. In the British historian's view, this may have been partly connected with a deterioration of the economic situation that produced a large unemployed population, such as landless peasants. Pay for service was disbursed annually at a rate of 5–20 solidi. The supply of weapons and uniforms was a state responsibility, and under Justinian a state monopoly was established over this activity.
The Byzantine navy in the 5th–6th centuries
Little is known about the West Roman and Byzantine navy in this period, possibly owing to the absence of major naval battles. Under the rule of the Vandal king Geiseric in the mid-5th century, Roman dominance of the Mediterranean Sea had been shaken, but the Byzantine fleet dominated the eastern Mediterranean, securing Constantinople and preventing incursions from Europe into Asia and vice versa. The Ostrogoths long lacked a strong fleet, and when in 508 the emperor Anastasius I (491–518) sent a fleet of 100 warships and the same number of dromons to the shores of Italy, it met with no resistance. This campaign may have spurred the development of an Ostrogothic navy. The navy is mentioned in connection with the actions in 515 of the general Vitalian, who rose up against the emperor Anastasius I. The rebels managed to assemble 200 ships in the ports of Thrace. Despite initial successes, their fleet was defeated by the praetorian prefect Marinus, who made active use of incendiary substances during the fighting.