This history of the Byzantine Empire covers the history of the Eastern Roman Empire from late antiquity until the Fall of Constantinople in 1453 AD. Several events from the 4th to 6th centuries mark the period during which the Roman Empires east and west divided. In 285, the emperor Diocletian partitioned the Roman Empires administration into eastern and western halves, between 324 and 330, Constantine I transferred the main capital from Rome to Byzantium, later known as Constantinople and Nova Roma. Under Theodosius I, Christianity became the Empires official state religion, and finally, under the reign of Heraclius, the Empires military and administration were restructured and adopted Greek for official use instead of Latin. The borders of the Empire evolved significantly over its existence, as it went through cycles of decline. During the reign of Maurice, the Empires eastern frontier was expanded, in a matter of years the Empire lost its richest provinces, Egypt and Syria, to the Arabs. This battle opened the way for the Turks to settle in Anatolia as a homeland, the final centuries of the Empire exhibited a general trend of decline. Its remaining territories were annexed by the Ottomans over the 15th century. The Fall of Constantinople to the Ottoman Empire in 1453 finally ended the Roman Empire, during the 3rd century, three crises threatened the Roman Empire, external invasions, internal civil wars and an economy riddled with weaknesses and problems. The city of Rome gradually became important as an administrative centre. The crisis of the 3rd century displayed the defects of the system of government that Augustus had established to administer his immense dominion. His successors had introduced some modifications, but events made it clearer that a new, more centralized, Diocletian was responsible for creating a new administrative system. He associated himself with a co-emperor, or Augustus, each Augustus was then to adopt a young colleague, or Caesar, to share in the rule and eventually to succeed the senior partner. After the abdication of Diocletian and Maximian, however, the tetrachy collapsed, Constantine moved the seat of the Empire, and introduced important changes into its civil and religious constitution. Constantine also began the building of the fortified walls, which were expanded. Constantine built upon the administrative reforms introduced by Diocletian and he stabilized the coinage, and made changes to the structure of the army. Under Constantine, the Empire had recovered much of its military strength and he also reconquered southern parts of Dacia, after defeating the Visigoths in 332, and he was planning a campaign against Sassanid Persia as well. In the course of the 4th century, four great sections emerged from these Constantinian beginnings, Constantine established the principle that emperors should not settle questions of doctrine, but should summon general ecclesiastical councils for that purpose
Map of the Roman Empire showing the four Tetrarchs' zones of influence after Diocletian's reforms.
Leo I of the Byzantine Empire (401–474, reigned 457–474)