History of the Roman Empire
Theodosius was the last Emperor who ruled over the whole Empire. After his death in 395, he gave the two halves of the Empire to his two sons Arcadius and Honorius; Arcadius became ruler in the East, with his capital in Constantinople, and Honorius became ruler in the West, with his capital in Milan and later Ravenna. The Roman state would continue to have two different emperors with different seats of power throughout the 5th century, though the Eastern Romans considered themselves to be the only ones who were fully Roman. Latin was used in official writings as much as, if not more than, Greek and the two halves were nominally, culturally and historically, if not politically, the same state.
Byzantine Empire
Theodosius I (379–395) was the last Emperor to rule both the Eastern and Western halves of the Empire. In 391 and 392 he issued a series of edicts essentially banning pagan religion. Pagan festivals and sacrifices were banned, as was access to all pagan temples and places of worship. The last Olympic Games are believed to have been held in 393. In 395, Theodosius I bequeathed the imperial office jointly to his sons: Arcadius in the East and Honorius in the West, once again dividing Imperial administration. In the 5th century the Eastern part of the empire was largely spared the difficulties faced by the West – due in part to a more established urban culture and greater financial resources, which allowed it to placate invaders with tribute and pay foreign mercenaries. This success allowed Theodosius II to focus on the codification of Roman law and further fortification of the walls of Constantinople, which left the city impervious to most attacks until 1204. Large portions of the Theodosian Walls are preserved to the present day.