 The "East Roman" (or Late Antique) era of Byzantium begins at the latest with the division of the Roman Empire into a Western Roman Empire and an Eastern Roman Empire (395).This "Early Byzantine" period lasts until approximately 641 AD.Emperor Justinian I (527–65) reconquered Italy, north Africa, and southern Spain, but after the expansion of Islam (634/98) a reorganized Byzantium, now based on administration by Themes, was limited to the Greek-speaking regions of the Balkan peninsula, Asia Minor, and southern Italy; Latin was abandoned as the language of officialdom.