Those differences between Eastern and Western social structures, together with certain geographical features, account for the different reception found by the Germanic invaders of the 4th and 5th centuries in the East and the West. Although the Germanic people had eddied about the Danube and Rhine frontiers of the empire since the 2nd century, their major inroads were made only in the latter half of the 4th century, when the ferocious Huns drove the Ostrogoths and Visigoths to seek refuge within the Danubian frontier of the empire. The initial interaction between Roman and barbarian was far from amicable; the Romans seemed to have exploited their unwelcome guests, and the Goths rose in anger, defeating an East Roman army at Adrianople in 378 and killing the Eastern emperor in command. Emperor Theodosius I (ruled 384–395) adopted a different policy, granting the Goths lands and according them the legal status of allies, or foederati, who fought within the ranks of the Roman armies as autonomous units under their own leaders.

Neither in West nor East did Theodosius’s policy of accommodation and alliance prove popular. The Goths—like most Germanic peoples, with the exception of the Franks and the Lombards—had been converted to Arian Christianity. Roman Christians considered Arianism a dangerous heresy, despite occasional imperial support, in the wake of the Council of Nicaea (325) and the Council of Constantinople (381) because of its emphasis upon the uniqueness of God the Father and subordination of the other two persons of the Trinity. The warlike ways of the Germans found little favour with a senatorial aristocracy essentially pacifist in its outlook, and the early 5th century is marked in both halves of the empire by reactions against Germanic leaders in high office. At Constantinople in 400, for example, the citizens rose against the senior officer of the imperial guard (magister militum), Gainas, slaughtering him together with his Gothic followers. Although that particular revolt was, in many respects, less productive of immediate results than similar episodes in the West, and the Germanic leaders later reappeared in roles of command throughout the East, the latter acted thenceforth as individuals without the support of those nearly autonomous groups of soldiers that western barbarian commanders continued to enjoy.

Furthermore, the East made good use of its resources in gold, in native manpower, and in diplomacy while quickly learning how best to play off one enemy against another. In the reign of Theodosius II (408–450), the Huns under their chieftain Attila received subsidies of gold that both kept them in a state of uneasy peace with the Eastern Empire and may have proved profitable to those merchants of Constantinople who traded with the barbarians. When Marcian (ruled 450–457) refused to continue the subsidies, Attila was diverted from revenge by the prospect of conquests in the West. He never returned to challenge the Eastern Empire, and, with his death in 453, his Hunnic empire fell apart. Both Marcian and his successor, Leo I (ruled 457–474), had ruled under the tutelage of Flavius Ardaburius Aspar, but Leo resolved to challenge Aspar’s preeminence and the influence of the Goths elsewhere in the empire by favouring the warlike Isaurians and their chieftain, Tarasicodissa, whom he married to the imperial princess, Ariadne. The Isaurian followers of Tarasicodissa, who was to survive a stormy reign as the Emperor Zeno (474–491), were rough mountain folk from southern Anatolia and culturally probably even more barbarous than the Goths or the other Germans. Yet, in that they were the subjects of the Roman emperor in the East, they were undoubtedly Romans and proved an effective instrument to counter the Gothic challenge at Constantinople. In the prefecture of Illyricum, Zeno ended the menace of Theodoric the Amal by persuading him (488) to venture with his Ostrogoths into Italy. The latter province lay in the hands of the German chieftain Odoacer, who in 476 had deposed Romulus Augustulus, the last Roman emperor in the West. Thus, by suggesting that Theodoric conquer Italy as his Ostrogothic kingdom, Zeno maintained at least a nominal supremacy in that western land while ridding the Eastern Empire of an unruly subordinate.

With Zeno’s death and the accession of the Roman civil servant Anastasius I (ruled 491–518), Isaurian occupation of the imperial office ended, but it was not until 498 that the forces of the new emperor effectively took the measure of Isaurian resistance. After the victory of that year, the loyal subject of the Eastern Roman emperor could breathe easily: Isaurians had been used to beat Germans, but the wild mountain folk had, in their turn, failed to take permanent possession of the imperial office. Imperial authority had maintained its integrity in the East while the Western Empire had dissolved into a number of successor states: the Angles and Saxons had invaded Britain as early as 410; the Visigoths had possessed portions of Spain since 417; the Vandals had entered Africa in 429; the Franks, under Clovis I, had begun their conquest of central and southern Gaul in 481; and Theodoric was destined to rule in Italy until 526.

Christological controversies

If ethnic hostility within the empire was less a menace about the year 500 than it had often been in the past, dissensions stemming from religious controversy seriously threatened imperial unity, and the political history of the next century cannot be understood without some examination of the so-called Nestorian and monophysite controversies. Following the dispute over Arian Christology (the doctrine of Christ), those disputes became stigmatized as the great heresies to afflict the Eastern Empire. If the Church Fathers of the 4th century quarreled over the relations between God the Father and God the Son, those of the 5th century faced the problem of defining the relationship of the two natures—the human and the divine—within God the Son, Christ Jesus.

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After Trinitarian Christianity became widely accepted as orthodoxy, Eastern Christological dispute was centred in two cities: Alexandria and Antioch. The theologians of Alexandria generally held that the divine and human natures were united indistinguishably within a single nature, though the questions of how they were related and whether in fact they were distinguishable were not settled immediately. The theologians of Antioch taught that two natures coexisted separately in Christ, the latter being “the chosen vessel of the Godhead…the man born of Mary.” In the course of the 5th century, those two contrasting theological positions became the subject of a struggle for supremacy between the rival sees of Constantinople, Alexandria, and Rome. When Nestorius, patriarch of Constantinople in 428, adopted the Antiochene formula in his argument that the Virgin Mary could not rightly be called Theotokos (literally “God-bearer”), or the mother of Christ’s divine nature, he was perceived as stressing the human nature of Christ to the neglect of the divine. His opponents—first the Alexandrian patriarch, Cyril, and later Cyril’s successor, Dioscorus, and a monk named Eutyches—in reaction stressed Christ’s divinity and its relationship with Christ’s human nature through the Incarnation. Cyril and Dioscorus became exemplary advocates of the Christological position called miaphysitism, which held, in Cyril’s words, that in Christ the human and the divine were incarnated within a single (Greek mia) nature (physis). Eutyches took the more-radical position, in denouncing so-called Nestorianism, that Christ’s divinity was greatly more significant than, and overwhelmed, his human nature. Neither Cyril nor Dioscorus held that position, and with the latter’s approval Eutyches was anathematized. Eutyches soon convinced Dioscorus (apparently via subterfuge) that he had seen the error of his ways and recanted his perspective on Christ’s humanity; Dioscorus subsequently endorsed Eutyches (causing some controversy) for reinstatement to the Christian fold.

Meanwhile, the claim of the Roman church was made by Pope Leo I, who in contrast declared for dyophysitism—i.e., the Christological position that two natures, perfect and perfectly distinct, existed in the single person of Christ. That struggle for power and legitimacy between Antioch, Alexandria, and Rome came to a head at the Council of Chalcedon (451). There the pope’s view triumphed, thanks to the support of Constantinople, which condemned both Nestorius for his extreme emphasis on the human nature of Christ and Eutyches (and, by extension, Dioscorus) for Eutyches’ purported monophysitism.

Chalcedon produced an indelible effect on Christian history beyond its immediate impact upon the purported orthodoxy of the Chalcedonian churches affiliated with Rome and Constantinople. The miaphysite, or non-Chalcedonian, churches—particularly the Coptic (Egyptian) and Syrian churches within the empire—were stigmatized as heretics, a situation that was not resolved until formal discussions in the late 20th century resolved many of the ancient disputes. (Ironically, both Chalcedonian and non-Chalcedonian churches invoked Cyril in their claims to Christian orthodoxy.)

More important for the purposes of military and political history than the theological details of the conflict was the impact miaphysitism produced on the several regions of the Mediterranean world. Partly because it provided a formula to express resistance to Constantinople’s imperial rule, miaphysitism persisted in Egypt and Syria. Until those two provinces were lost to Islam in the 7th century, each Eastern emperor had to somehow cope with their separatist tendencies as expressed in the heresy. He had either to take arms against miaphysitism and attempt to extirpate it by force, to formulate a creed that would somehow blend it with dyophysitism, or to frankly adopt this position as his own belief. None of those three alternatives proved successful, and religious hostility was not the least of the disaffections that led Egypt and Syria to yield, rather readily, to the Arab conqueror. If ever the East Roman emperor was to reassert his authority in the West, he necessarily had to discover a formula that would satisfy Western orthodoxy while not alienating non-Chalcedonian Christians.

The empire at the end of the 5th century

In the reign of Anastasius I (491–518), all those tendencies of the 5th century found their focus: the sense of Romanitas, which demanded a Roman rather than an Isaurian or a German emperor, the conflict between Chalcedonian and non-Chalcedonian churches, and the persisting economic prosperity of the Eastern Roman Empire. Acclaimed and elected as the Roman and Orthodox emperor who would end both the hated hegemony of the Isaurians and the detested activity of the purported monophysites, Anastasius succeeded in the first of those objectives while failing in the second. While he defeated the Isaurians and transported many of them from their Anatolian homeland into Thrace, he gradually came to support non-Chalcedonian Christianity despite the professions of Orthodoxy he had made upon the occasion of his coronation. If his policies won him followers in Egypt and Syria, they alienated his Orthodox subjects and led, finally, to constant unrest and civil war.

Anastasius’s economic policies were far more successful; if they did not provide the basis for the noteworthy achievements of the 6th century in military affairs and the gentler arts of civilization, they at least explain why the Eastern Empire prospered in those respects during the period in question. An inflation of the copper currency, prevailing since the age of Constantine, finally ended with welcome results for those members of the lower classes who conducted their operations in the base metal. Responsibility for the collection of municipal taxes was taken from the members of the local senate and assigned to agents of the praetorian prefect. Trade and industry were probably stimulated by the termination of the chrysargyron, a tax in gold paid by the urban classes. If, by way of compensating for the resulting loss to the state, the rural classes had then to pay the land tax in money rather than kind, the mere fact that gold could be presumed to be available in the countryside is a striking index of rural prosperity. In the East, the economic resurgence of the 4th century had persisted, and it is not surprising that Anastasius enriched the treasury to the extent of 320,000 pounds of gold during the course of his reign.

With such financial resources at their disposal, the emperor’s successors could reasonably hope to reassert Roman authority among the western Germanic successor states, provided they could accomplish two objectives: first, they had to heal the religious discord between their subjects; second, they had to protect the eastern frontier against the threat of Sāsānian Persia. Since there was, in fact, to be concurrent warfare on both fronts during the 6th century, some knowledge of the age-old rivalry between Rome and Persia is essential to an understanding of the problems confronted by the greatest among Anastasius’s successors, Justinian I (ruled 527–565), as he undertook the conquest of the West.

In 224 the ancient Persian empire had passed into the hands of a new dynasty, the Sāsānians, whose regime brought new life to the enfeebled state. Having ensured firm control over the vast lands already subject to them, the Sāsānians took up anew the old struggle with Rome for northern Mesopotamia and its fortress cities of Edessa and Nisibis, lying between the Tigris and the Euphrates. In the course of the 4th century, new sources of hostility emerged as East Rome became a Christian empire. Partly by reaction, Sāsānian Persia strengthened the ecclesiastical organization that served its Zoroastrian religion; intolerance and persecution became the order of the day within Persia, and strife between the empires assumed something of the character of religious warfare. Hostilities were exacerbated when Armenia, lying to the north between the two realms, converted to Christianity and thus seemed to menace the religious integrity of Persia. If small-scale warfare during the 4th and 5th centuries rarely erupted into major expeditions, the threat to Rome nonetheless remained constant, demanding vigilance and the construction of satisfactory fortifications. By 518, the balance might be said to have tipped in the favour of Persia as it won away the cities of Theodosiopolis, Amida, and Nisibis.