Byzantine army in the context of "Varangians"

In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate. During the period 395–476, the army of the Roman Empire's western half progressively disintegrated, while its counterpart in the East, known as the East Roman army (or the early Byzantine army) remained largely intact in size and structure until the reign of Justinian I (r. AD 527–565).

The Imperial Roman army of the Principate (30 BC – 284 AD) underwent a significant transformation as a result of the chaotic 3rd century. Unlike the army of the Principate, the army of the 4th century was heavily dependent on conscription and its soldiers were paid much less than in the 2nd century. Barbarians from outside the empire probably supplied a much larger proportion of the late army's recruits than in the army of the 1st and 2nd centuries, but there is little evidence that this adversely affected the army's combat performance.

The Battle of Manzikert or Malazgirt was fought between the Byzantine Empire and the Seljuk Empire on 26 August 1071 near Manzikert, Iberia (modern Malazgirt in Muş Province, Turkey). The decisive defeat of the Byzantine army and the capture of the emperor Romanos IV Diogenes played an important role in undermining Byzantine authority in Anatolia and Armenia, and allowed for the gradual Turkification of Anatolia. Many Turks, travelling westward during the 11th century, saw the victory at Manzikert as an entrance to Asia Minor.

The brunt of the battle was borne by the Byzantine army's professional soldiers from the eastern and western tagmata, as large numbers of mercenaries and Anatolian levies fled early and survived the battle. The fallout from Manzikert was disastrous for the Byzantines, resulting in civil conflicts and an economic crisis that severely weakened the Byzantine Empire's ability to defend its borders adequately. This led to the mass movement of Turks into central Anatolia – by 1080, an area of 78,000 square kilometres (30,000 sq mi) had been gained by the Seljuk Turks. It took three decades of internal strife before Alexius I (1081–1118) restored stability to Byzantium. Historian Thomas Asbridge says: "In 1071, the Seljuqs crushed an imperial army at the Battle of Manzikert (in eastern Asia Minor), and though historians no longer consider this to have been an utterly cataclysmic reversal for the Greeks, it still was a stinging setback." It was the only time a Byzantine emperor became the prisoner of a Muslim commander, and the first time since Valerian that a Roman emperor was captured alive by an enemy force.

The Palaiologan army refers to the military forces of the Byzantine Empire under the rule of the Palaiologos dynasty, from the late 13th century to its final collapse in the mid-15th century. The army was a direct continuation of the forces of the Empire of Nicaea, which itself was a fractured component of the formidable Komnenian army of the 12th century. 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 Komnenos, were boosted to include 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.

After Andronikos II took to the throne in 1282, 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 Latin territories in Greece. By c. 1350 the Empire's inefficient fiscal organization and incompetent central government made raising troops and the supplies to maintain them a near-impossible task, and the Empire came to rely upon troops provided by Serbs, Bulgarians, Venetians, Latins, Genoese and 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. By the time the civil war had ended, the Turks had cut off Constantinople, the capital of the Byzantine Empire, from the surrounding land and in 1453 the last decisive battle was fought by the Palaiologan army when the capital was stormed and sacked, falling on 29 May.

The Battle of Adrianople also known as Battle of Hadrianopolis was fought between the Eastern Roman army led by the Roman emperor Valens and Gothic rebels (largely Thervings as well as Greutungs, non-Gothic Alans, and various local rebels) led by Fritigern. The battle took place on 9 August 378 in the vicinity of Adrianople, in the Roman province of Thracia (modern Edirne in European Turkey). It ended with an overwhelming victory for the Goths and the death of Emperor Valens.

As part of the Gothic War of 376–382, the battle is often considered the start of the events which led to the fall of the Western Roman Empire in the 5th century.

In the year before the First Council of Constantinople in 381, Nicene Christianity became the official religion of the Roman Empire when Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II issued the Edict of Thessalonica in 380, which recognized the catholic orthodoxy, as defined by the Council of Nicea, as the Roman Empire's state religion. Historians refer to this state-sponsored church using a variety of terms: the catholic church, the orthodox church, the imperial church, the Roman church, or the Byzantine church, with some also used for wider communions extending beyond the Roman Empire. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church all claim to stand in continuity from the Nicene Christian church to which Theodosius granted recognition. Political differences between the Eastern Roman Empire and the Persian Sassanid Empire led to the separation of the Church of the East in 424. A doctrinal split within the imperial church led to the independence of early Oriental Orthodoxy, while the fall of the Western Roman Empire initiated the gradual separation between Eastern and Western Christianity, culminating in the East-West schism of 1054. The Western Church evolved into the Latin Catholic Church while the Eastern Church that remained under the patronage of the Eastern empire and evolved into the Greek Orthodox Church.

Earlier in the 4th century, following the Diocletianic Persecution of 303–313 and the Donatist controversy that arose in consequence, Constantine the Great had convened councils of bishops to define the orthodoxy of the Christian faith and to expand on earlier Christian councils. A series of ecumenical councils convened by successive Roman emperors met during the 4th and the 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the theological and christological doctrines of Arianism, Nestorianism, Miaphysitism, and Dyophysitism. In the 5th century, the Western Roman Empire decayed as a polity; invaders sacked Rome in 410 and in 455, and Odoacer, an Arian barbarian warlord, forced Romulus Augustus, the last nominal Western Emperor, to abdicate in 476. However, apart from the aforementioned schisms, the church as an institution persisted in communion, if not without tension, between the East and West. In the 6th century, the Byzantine armies of the Byzantine Emperor Justinian I recovered Italy and other regions of the Western Mediterranean shore. The Byzantine Empire soon lost most of these gains, but it held Rome, as part of the Exarchate of Ravenna, until 751, a period known in church history as the Byzantine Papacy. The early Muslim conquests of the 7th–9th centuries would begin a process of converting most of the then-Christian world in the Levant, Middle East, North Africa, regions of Southern Italy and the Iberian Peninsula to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Christian missionary activity directed from the capital of Constantinople did not lead to a lasting expansion of the formal link between the church and the Byzantine emperor, since areas outside the Byzantine Empire's political and military control set up their own distinct churches, as in the case of Bulgaria in 919.

The Eastern Roman army refers to the army of the eastern section of the Roman Empire, from the empire's definitive split in 395 AD to the army's reorganization by themes after the permanent loss of Syria, Palestine and Egypt to the Arabs in the 7th century during the Byzantine-Arab Wars. The East Roman army was the continuation of the Late Roman army of the 4th century, until it gradually transformed into what is now called the Byzantine army from the 7th century onwards.

The East Roman army was a direct continuation of the eastern portion of the late Roman army, from before the division of the empire. The East Roman army started with the same basic organization as the late Roman army and its West Roman counterpart, but between the 5th and 7th centuries, the cavalry became more important, the field armies took on more tasks, and the border armies were transformed into local militias.

Michael VIII Palaiologos or Palaeologus (Greek: Μιχαὴλ Δούκας Ἄνγελος Κομνηνὸς Παλαιολόγος, romanized: Mikhaḗl Doúkās Ángelos Komnēnós Palaiológos; 1224 – 11 December 1282) reigned as Byzantine emperor from 1261 until his death in 1282, and previously as the co-emperor of the Empire of Nicaea from 1259 to 1261. Michael VIII was the founder of the Palaiologan dynasty that would rule the Byzantine Empire until the Fall of Constantinople in 1453. He recovered Constantinople from the Latin Empire in 1261 and transformed the Empire of Nicaea into a restored Byzantine Empire. His reign saw considerable recovery of Byzantine power, including the enlargement of the Byzantine army and navy. It also included the reconstruction of the city of Constantinople, and the increase of its population. His re-establishment of the University of Constantinople contributed to the Palaeologan Renaissance, a cultural flowering between the 13th and 15th centuries.

It was also at this time that the focus of the Byzantine military shifted to the Balkans, against the Bulgarians, leaving the Anatolian frontier neglected. His successors could not compensate for this change of focus, and both the Arsenite schism and two civil wars which occurred from 1321–1328 and 1341–1347 undermined further efforts toward territorial consolidation and recovery, draining the empire's strength, economy, and resources. Regular conflict between Byzantine successor states such as Trebizond, Epirus, Bulgaria and Serbia resulted in permanent fragmentation of former Byzantine territory and opportunity for increasingly successful conquests of expansive territories by post-Seljuk Anatolian beyliks, most notably that of Osman, later called the Ottoman Empire.

The limitanei (Latin, also called ripenses), meaning respectively "the soldiers in frontier districts" (from the Latin word limes meaning frontier) or "the soldiers on the riverbank" (from the Rhine and Danube), were an important part of the late Roman and early Byzantine army after the reorganizations of the late 3rd and early 4th centuries. The limitanei, unlike the comitatenses, palatīni, and scholae, garrisoned fortifications along the borders of the Roman Empire and were not normally expected to fight far from their fortifications.

The limitanei were lower-status and lower-paid than the comitatenses and palatīnī, and the distinction in role and status between scholae, palatini, comitatenses, and limitanei had largely replaced the older one between praetorians, legionaries, and auxiliaries. The limitanei and palatini both included legionary units alongside auxiliary units.