The events of Zeno's reign are obscure and complex. The only continuous narrative to have survived is by Evagrius, in book three of his Ecclesiastical History. Zeno's rule overlapped with the final collapse of the empire in the West, and the fates of the two halves of the Roman world were not independent of one another. Leading roles were played in the east by Armatus, who was a critical figure in the power struggles at Constantinople, and in the west by Odoacar, who ruled the rump of what had been the western empire after the deposition of Romulus Augustulus.
Zeno's accession to the eastern throne in 474 provoked an immediate conspiracy involving Verina, the widow of the previous emperor Leo, her brother, the patricius Basiliscus, their nephew Armatus, magister utriusque militiae for Thrace, and Patricius, a former magister officiorum.50 For a two-year period Zeno was driven from his capital by this rival group, who were aided by the Ostrogoths of Thrace under Theoderic Strabo. After 476 Zeno was reinstated, but failed to establish full control, having bargained away much of his power to his rivals. He called upon the Ostrogothic group led by Theoderic the Amal to do battle with the Thracian group under Theoderic Strabo. However, the Gothic leaders saw through this strategy of divide and rule. As Theoderic the Amal put the situation to his namesake,
While remaining at peace the Romans expect the Goths to wear each other down. Whichever of us fails, they will be the winners with none of the effort, and whichever of us destroys the other side will benefit only from a Cadmeian victory and be left in diminished numbers to face Roman treachery. (Malchus fr. 18.2)
Theoderic Strabo now joined forces with the Amal. Their forces advanced towards Constantinople but were repulsed at the long walls across Thrace. Zeno made terms with Strabo and managed initially to outwit Theoderic the Amal. By the end of 479 Theoderic the Amal took refuge on the coast of Epirus, where Alaric had been based eighty years before, but was at war with Zeno and Theoderic Strabo.
Zeno's own position throughout his reign was weakened by rivalry with Illus, a fellow Isaurian, who had the support of many of the Isaurian troops in Constantinople, but the two united in successfully resisting a coup by Marcianus, grandson of the emperor of 450–7. Theoderic Strabo may have been party to the plot; at all events he was replaced as commander of the garrison army by Illus' brother Trocondes. Deprived of his livelihood and status, Theoderic Strabo attacked Greece, but died as a result of a riding accident on the road between Philippi and Maximianopolis (Evagrius, HE 3.25). Meanwhile Theoderic the Amal accepted new overtures from Zeno to support him against Illus, and was appointed magister utriusque militiae praesentalis and made consul for 484, the first time that a barbarian from outside the empire reached this rank.
After 484 most of the Thracian Goths joined Theoderic the Amal or enlisted in forces which were sent to fight in the East. The dispute between Zeno and Illus now became more acute and led to a civil war. Zeno defeated Illus at Antioch in September 484, and then besieged him at the Isaurian fortress of Papirion, where he managed to resist until 488. The involvement of the powerful Isaurian factions had embroiled Asia Minor, at the heart of the eastern empire, in open warfare for the first time since the episode of the Gothic leaders Tribigild and Gainas in 400.51
During this war doubts were raised about the loyalty of Theoderic the Amal, and in 486 he again revolted against Zeno, advancing from Thrace to Constantinople and cutting off the city's water supply. Zeno paid off Theoderic after negotiations, and it was mutually decided that the Ostrogoths should go to Italy to recover the situation in the West, where no emperor had ruled since the fall of Romulus Augustulus, ten years earlier. There is a clear comparison to be made here between the roles played by Theoderic and by Alaric at the end of the 390s, when the latter was given effective command of Roman as well as his own forces in Illyricum, and, with or without direct instructions from the eastern empire, invaded Italy and attacked Stilicho in 402.52
Odoacar, the future king of Italy, and Onoulph were two sons of the union of Ediko, Attila's trusted legate, and his Scirian wife. They rose to powerful positions between the death of Attila in 453 and the battle of the river Bolia in 469, when the remnant portion of the declining Hunnic Empire finally yielded to the emergent Ostrogoths. Onoulph now came to Constantinople, while Odoacar turned west. Odoacar was active in Noricum around 470,53 and is attested as a bodyguard of Ricimer in Italy during his struggle with Anthemius (Procopius, Bell. Goth. 1.6). By 476 he was the leader of a mixed group of barbarians fighting in Italy, who declared him king on August 23, 476 (Jordanes, Get. 242, 291). Within a fortnight he had killed Orestes and banished the last emperor Romulus from the palace in Ravenna to an estate on the bay of Naples (Anon. Val. 8.37–8). This takeover coincided with the coup of Basiliscus against Zeno in Constantinople. Meanwhile Zeno, now supported by Basiliscus' former associate Armatus, had regained power in the East, and Odoacar obliged Romulus Augustulus, as his last act, to send a senatorial embassy to Zeno, proposing that no second emperor was now required in the West but that Odoacar should be given the rank of patricius to wield authority on the Roman behalf. This coincided at Constantinople with an embassy from the almost forgotten figure of Julius Nepos, who had been appointed Augustus by Leo in 474. Zeno proposed that Nepos should be recognized as emperor, with Odoacar as his patricius (Malchus fr. 14).
Nepos was murdered in 480, but Odoacar brought a decade of relative stability to Italy, and to Dalmatia, which was now attached to it. Gaul was largely left to its own fate. Between the regions controlled by the Merovingian Franks, the Visigoths and the Burgundians there were two Roman enclaves, in mid-France around Soissons, and in Provence around Arles. From 457 to his death in 464 the effective leader of this Roman territory was Aegidius, magister militum per Gallias, who with Merovingian help managed to reclaim Lyon from the Burgundians and defend Arles from the Visigoths. He was succeeded in 469 by his son Syagrius. Gregory of Tours indicates that both Aegidius and his son were treated as kings rather than as Roman officials by the Franks (Greg. Tur., Hist. II. 12, 27), and Syagrius was killed in 486 at the battle of Soissons by Merovingian forces under King Clovis.
In 488 Zeno reached his understanding with Theoderic the Amal, that the latter should take his Ostrogothic followers away from Thrace to overthrow Odoacar in Italy. Open warfare, with victories on either side extended from August 488 until Theoderic, supported by Visigothic forces sent from Toulouse by Alaric II, defeated Odoacar at the battle of Adda on August 11, 490, and besieged his rival in Ravenna (Anon. Val. 11.50–56). After a three-year siege an agreement was negotiated by the city's bishop, according to which the two leaders would rule Italy in concert. On March 5 493, Theoderic entered the city; and on the Ides of March he murdered Odoacar with his own hands. He was to rule Italy from Ravenna until 526.