linguistic:translation
Early history (pre-518)

In a series of conflicts between the third and first centuries BC, the Roman Republic gradually established hegemony over the eastern Mediterranean, while its government ultimately transformed into the one-person rule of an emperor. The Roman Empire enjoyed a period of relative stability until the third century AD, when a combination of external threats and internal instabilities caused the Roman state to splinter as regional armies acclaimed their generals as "soldier-emperors". One of these, Diocletian (r. 284–305), seeing that the state was too big to be ruled by one man, attempted to fix the problem by instituting a Tetrarchy, or rule of four, and dividing the empire into eastern and western halves. Although the Tetrarchy system quickly failed, the division of the empire proved an enduring concept.

Constantine I (r. 306–337) secured sole power in 324. Over the following six years, he rebuilt the city of Byzantium as a capital city, which was renamed Constantinople. Rome, the previous capital, was further from the important eastern provinces and in a less strategically important location; it was not esteemed by the "soldier-emperors" who ruled from the frontiers or by the empire's population who, having been granted citizenship, considered themselves "Roman". Constantine extensively reformed the empire's military and civil administration and instituted the gold solidus as a stable currency. He favoured Christianity, which he had converted to in 312. Constantine's dynasty fought a lengthy conflict against Sasanid Persia and ended in 363 with the death of his son-in-law Julian. The short Valentinianic dynasty, occupied with wars against barbarians, religious debates, and anti-corruption campaigns, ended in the East with the death of Valens at the Battle of Adrianople in 378.

Valens's successor, Theodosius I (r. 379–395), restored political stability in the east by allowing the Goths to settle in Roman territory; he also twice intervened in the western half, defeating the usurpers Magnus Maximus and Eugenius in 388 and 394 respectively. He actively condemned paganism, confirmed the primacy of Nicene Christianity over Arianism, and established Christianity as the Roman state religion. He was the last emperor to rule both the western and eastern halves of the empire; after his death, the West would be destabilised by a succession of "soldier-emperors", unlike the East, where administrators would continue to hold power. Theodosius II (r. 408–450) largely left the rule of the east to officials such as Anthemius, who constructed the Theodosian Walls to defend Constantinople, now firmly entrenched as Rome's capital.

Theodosius' reign was marked by the theological dispute over Nestorianism, which was eventually deemed heretical, and by the formulation of the Codex Theodosianus law code. It also saw the arrival of Attila 's Huns, who ravaged the Balkans and exacted a massive tribute from the empire; Attilla however switched his attention to the rapidly-deteriorating western empire, and his people fractured after his death in 453. After Leo I (r. 457–474) failed in his 468 attempt to reconquer the west, the warlord Odoacer deposed Romulus Augustulus in 476, killed his titular successor Julius Nepos in 480, and the office of western emperor was formally abolished.

Through a combination of luck, cultural factors, and political decisions, the Eastern empire never suffered from rebellious barbarian vassals and was never ruled by barbarian warlords—the problems which ensured the downfall of the West. Zeno (r. 474–491) convinced the problematic Ostrogoth king Theodoric to take control of Italy from Odoacer, which he did; dying with the empire at peace, Zeno was succeeded by Anastasius I (r. 491–518). Although his Monophysitism brought occasional issues, Anastasius was a capable administrator and instituted several successful financial reforms including the abolition of the chrysargyron tax. He was the first emperor to die with no serious problems affecting his empire since Diocletian.