At the death of Theodosius in 450 no clear successor was in view, and Pulcheria was virtually sole empress for a month-long interregnum. She and the magister militum Aspar resolved the issue by choosing a Thracian military officer Marcian, who had served under Aspar and now married Pulcheria.35 After the removal of Chrysaphius, Marcian called another ecumenical council, held at Chalcedon in 451, which reversed the decisions of Ephesus and asserted the new orthodox doctrine of the dual nature of Christ, that he was both fully human and fully divine in his two natures. The rugged church politics displayed at Chalcedon indicate some significant realignments in ecclesiastical authority. Leo the bishop of Rome, who passionately opposed any hint of Monophytism, engaged with the issues on the side of Constantinople as no western church leader had done before. The growing rivalry between Constantinople and Alexandria culminated in the decision that the patriarchate of Constantinople should rank second in the Christian world after Rome.36 The enforcement of the decisions taken at Chalcedon was the result of a coalition between the secular state and the powerful bishops of Rome and Constantinople. This was the culmination of Theodosian policy in the first half of the fifth century.
Marcian enjoyed good fortune in his foreign policy. After the death of Attila, as his sons disputed with one another in feuds among the loose assemblage of tribes that he had controlled, the threat from the Huns gradually evaporated. Marcian discontinued the tribute payments which had drained Theodosius' Constantinople, and was able to reduce taxes to the approval of the Senate. By his death in 457 the treasury was reported to be in substantial surplus.37
Marcian was succeeded in February 457 by another senior military figure, Leo. According to the detailed account recorded by Constantine Porphyrogenitus in the tenth century, he was the first emperor to be raised aloft on the shields of his soldiers, in a gesture that recalled the practice of the Germanic troops that had proclaimed Julian Augustus in Paris in 360. He was then escorted to the church of the Holy Apostles, to be crowned by the bishop of Constantinople This was the first coronation at which the patriarch is recorded as playing this role, and this further confirmed the growing authority of the Constantinopolitan Church (Constantine Porphyrogenitus, De Caer. 1.91). The most powerful military figure in Constantinople was the general Aspar, of Alan origin, whose influence can be traced back to the 420s.
Leo's reign in the East and the bewildering succession to Valentinian III, who had been murdered at Ravenna in 455, heralded an era of great uncertainty and confusion in the empire as a whole. The balance of power was evenly divided between the eastern and western Roman courts and the major barbarian groups, especially the Vandals in Africa and the Ostrogothic groups, which began to dominate Illyricum as Hunnic power declined. In the East there was a new element, the rise of Isaurian leaders and their followers from the Taurus Mountains in southern Asia Minor, who were effectively another barbarian state located within the frontiers of the eastern empire. The interplay of these competing ethnic groups was to last for more than a generation, until the establishment of a new order by the long reigns of Anastasius in the East between 491 and 518, and of the Ostrogothic king Theoderic, who ruled from Ravenna after the collapse of the western empire from 493–526.
Leo's reign lasted until 474. Our best source for the events of the 450s and 460s after the fall of Attila is the Getica of Jordanes, which incorporates reliable detail from otherwise lost sections of Priscus' history. His account of the battle of river Nedao, at which the Germanic tribal groups began to gain the upper hand over the Huns, describes the anarchic situation:
There an encounter took place between the various nations Attila had held under his sway. Kingdoms with their peoples were divided, and out of one body were made many limbs not responding to a single impulse. Being deprived of their head, they madly strove against each other. They never found their equals ranged against them without harming each other by wounds mutually given. And so the bravest of nations tore themselves to pieces. (Jordanes, Get. L, 261, trans. Mierow = Priscus fr. 25)
This period saw the initial rise of the Ostrogoths, a tribal name first mentioned by Claudian in 399, when they were connected with the Greuthungi (Claudian, In Eutrop. 2, 158). The Ostrogothic tribes evolved as the dominant Germanic tribal peoples of the Balkans, clearly distinguished from their Visigothic cousins, who were settled in Aquitaine.
The Isaurians now also emerged to provide a counterweight to the Ostrogoths, particularly in the struggle for political control at Constantinople. Isauria, the mountainous area of southern Asia Minor between Iconium and Seleucia, had never been fully integrated into the structures and ideology of the Roman Empire. During the fourth and fifth centuries the region was in effect a large enclave of barbarian territory within the frontiers of the eastern empire, enclosed by a limes Isauricus.38 Civic life was barely established in the mountainous interior of the country, and the future imperial family came a from a village called Rusumblada, which was only later to achieve the title of a city, Zenonopolis. The elder Zeno had helped to defend Constantinople from Attila in 447, and even at that date was seen as a contender for imperial power. The Isaurians, unlike the Goths and other Germanic barbarian groups, were nominally Catholic Christians, not Arians, and thus not debarred from imperial rank on sectarian grounds. The future emperor Zeno, known by his native name Tarasikodissa, became a trusted intimate of the emperor Leo, and was promoted from being comes domesticorum to praetorian prefect of the East. He married Ariadne, Leo's daughter, and replaced his Isaurian nomenclature with the Greek name that had been used by Theodosius' formidable Isaurian magister militum, Zeno. The Isaurian group was heavily involved when the “dynasty of Aspar and his sons” fell under suspicion of treachery in 471 and were murdered in the palace. Aspar's surviving supporters fled to Thrace to join the Ostrogoths under Theoderic Strabo.39 There was resistance to Leo's attempt to designate Zeno emperor, but it was adroitly circumvented. When Leo died in 474, he was succeeded by Zeno and Ariadne's infant child, also called Leo, who reigned under his father's protection. The young Leo died a few months later leaving Zeno as sole emperor.