During the long reign of Theodosius II (408-450), the Huns had become a real threat to the heartland of the Eastern Roman Empire. Raids began in 441, in Pannonia as well as along the Danube, staved off only by Constantinople's consent to paying more tribute to the Hun leader Attila. In 447, Hun armies returned to imperial lands, in two simultaneous thrusts. While one went directly to Constantinople, the other thundered through Macedonia, as far south as Thermopylae in Epirus. Though the capital's walls prevented Hun penetration, the Huns did defeat an imperial army near Gallipoli. Attila extracted yet another increase in annual tribute from Theodosius, who died in 450.
Theodosius had produced no male heir, so his daughter Pulcheria married Marcian, a senator and retired officer from Thrace. Seen as unobjectionable by Constantinople elites, he was accepted as Emperor in 450. Aware that Attila was planning a western expedition, one of Marcian's first acts was to refuse the former's demand for more tribute. This was wildly popular, and the East was spared further Hun depredations when Attila led his hordes to Western Roman lands. The remainder of Marcian's rule (d. 457) was filled with religious disputes. Given his role as protector and de facto manager of the Church in the East, the Emperor could not avoid involvement.
The dispute which erupted in the early 450s was similar to the Arianism controversy of the 300s in that involved the nature of Christ and his relationship to the other parts of the Trinity. Back in 448 the cleric Eutyches had been accused of spreading the doctrine that Christ was not bot human and divine. Rather, being more powerful than human, Christ's divinity had overpowered his mortality. Thus, Christ possessed only a single nature. This made sense given Hellenistic philosophical assumptions of the East. This was called Monophysitism, and after its proponents appealed to friendly bishops as well as the Pope Leo I, it became an expanding crisis in the Christian world. In 451, the Fourth Ecumenical Council was held at Chalcedon. Including nearly 600 bishops, the council condemned Eutyches again (he had been previously reinstated) and articulated the Chalcedonian Definition: Christ was held to possess one person and two natures, "unconfusedly, unchangeably, indivisibly and inseparably" united. Though satisfactory in the West and to most Eastern clerics of the time, it made little sense to several clerics and laymen in Egypt, Syria, and other areas. Thus, Monophysitism continued in these areas, invigorating Nestorianism in the Levant as well as Coptic Christianity in Egypt. At times these regions would express their religious difference through separatism. Another noteworthy aspect of the Council was a decree that henceforth the Bishop of Constantinople would be a Patriarch, second only to the Rome Pope in the Christian hierarchy. In future years this led to an Eastern interpretation that Papal supremacy was purely nominal, with eastern provinces responsible to the Constantinople Patriarch alone. Church schism would ultimately result.
Again, in 457 there was no male heir, and no imperial candidate from the Theodosian line stretching back to the 370s. Stepping into the gap as emperor- maker was the Arian Alan serving as Master of Soldiers, Aspar. As both his religion and ethnicity were increasingly offensive to Eastern Romans, he remained the power behind the throne, appointing his household steward as Emperor Leo I (r. 457-474). The new Emperor soon chafed at Aspar's tutelage, and also sensed growing popular resentment against Germanic prevalence in the army especially. Leo thus decided to purge the military of Germans, and the remainder of his reign was occupied with the struggle against Aspar and his colleagues. These included Basiliscus, Leo's brother-in-law, who while a Hellenized Roman devoted to Monophysitism, shared in common with the adept yet Germanic general a hatred of the Emperors new allies. These were mostly Isaurians, battle-hardened mountain tribes from beyond the Taurus ranges in Southeastern Anatolia. Leo was particularly close to an important chieftain, Tarasicodissa Rousoumbladeotes, who modified his name to Zeno upon marrying Leo's daughter Ariadne.
Sparring between the two camps began in 468. Leo decided to launch a large- scale expedition against the Vandals in North Africa, and Basiliscus arranged his own command of it, to position himself for eventual imperial rule. After landing, however, Basiliscus did not take advantage of early victories by subordinates, and allowed Vandal leader Gaiseric to trick him by false offers of surrender. Instead, the Vandals prepared a fleet of fire-boats, with which they incinerated the Byzantine fleet. Basiliscus fled the scene prematurely, and was forced to seek refuge in St. Sophia in Constantinople. Aspar as well was tainted by the defeat. Attempting to even the scales, in 469 he plotted an assassination of Zeno, yet was thwarted at the last moment. In 471 his son Ardabur even tried to woe Isaurian clans away from Zeno. His patience exhausted, Leo had Aspar and his son killed the same year by palace guards.
Leo died in 474, having assured his people a spell of Orthodoxy and comparative peace. He had appointed as his successor his grandson and Zeno's son, Leo II. Ariadne prevailed upon the latter to immediately elevate Zeno to co-Emperor. Nine months later Leo died, and Zeno became sole ruler (474-491). His first measure was to negotiate a peace with the Vandals, who henceforth no longer threatened Byzantium. He spent the next decade dealing with severe challenges to the throne. Basiliscus and Ariadne's mother Verina were still the chief initial antagonists. Joining them was Illus, an Isaurian general whose motives are mysterious. With him, they appeared to gain the support of certain influential senators and segments of the population opposed to Isaurian presence on the throne. At the end of 475, Zeno felt sufficiently threatened to flee Constantinople for the Taurus mountains. Basiliscus appeared to realize his ambitions when he was subsequently proclaimed emperor. His rule until 477 was a disaster. The masses were alienated through harsh taxation, while the Church despised him for trying to impose Monophysitism in the Empire, going so far as to abrogate the Council of Chalcedon. Seeing public disaffection from Basiliscus, Illus returned to Zeno, readying him for a counter-attack. Basiliscus was finally undone when he made his own nephew Harmatius Master of Soldiers. A delusionary hedonist, he was easily persuaded to support Zeno by promise of government advancement. Thus, in 477 Zeno returned as Emperor, and Basiliscus was exiled to Cappadocia.