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.
History of Rome
Rome's population declined after its peak in the 2nd century. At the end of that century, during the reign of Marcus Aurelius, the Antonine Plague killed 2,000 people a day. Marcus Aurelius died in 180, his reign being the last of the "Five Good Emperors" and Pax Romana. His son Commodus, who had been co-emperor since AD 177, assumed full imperial power, which is most generally associated with the gradual decline of the Western Roman Empire. Rome's population was only a fraction of its peak when the Aurelian Wall was completed in the year 273 (in that year its population was only around 500,000).