Saylor.org's Ancient Civilizations of the World/The Eastern Roman Empire and Constantinople
When historians speak of the fall of the Roman Empire, technically they really mean the fall of the Western Empire. The Eastern Empire, with its emperor reigning from Constantinople, would continue to play (an ever-diminishing) role in the Mediterranean world for another millennium, until the fall of Constantinople to Turkish forces in 1453. The idea of the "Byzantine Empire" as an entity separate from the legacy of the Roman Empire is a largely historiographical one. Those who lived in the Empire throughout the period did not refer to it as the Byzantine Empire, rather they simply called it the "Roman Empire", or "Romania". To those living in the Empire, the state was formally the continuation of the Roman state, traditions and customs. However, even during the last century of the Western Roman Empire, the Byzantines began orienting themselves away from the traditions of Rome. Byzantium was to turn to Greek culture and language, as opposed to Latin. Even more important was its turn to what would become Eastern Orthodox Christianity instead of Roman paganism, or what would become Roman Catholicism.
Constantine the Great and the Beginning of Byzantium
It is a matter of debate when the Roman Empire officially ended and transformed into the Byzantine Empire. Most scholars accept that it did not happen at one time, but that it was a slow process, and so late Roman history overlaps with early Byzantine history.Constantine I (“the Great”) is usually held to be the founder of the Byzantine Empire. He was responsible for two major changes that would help create a Byzantine culture distinct from the Roman past. First, he legalized Christianity, which had previously been persecuted in the Roman Empire. He converted to Christianity, and sponsored the Christian Church. Christianity would be a major feature at the heart of Byzantine culture. The second major step taken by Constantine was moving the capital of the Roman Empire to the city of Byzantium (the origin of the word “Byzantine”), which he refounded as the city of Constantinople (it was also sometimes called “New Rome”. Constantine’s founding of Constantinople in 330 CE is usually considered the beginning of the Byzantine Empire. By moving the capital of the empire outside Italy and to the East, Constantine moved the center of gravity in the empire to this region, right on the divide between Europe and Asia Minor. After Constantine, few emperors ruled the entire Roman Empire. It was too big, and under attack from too many directions. Usually, there was an emperor of the Western Roman Empire ruling from Italy or Gaul, and an emperor of the Eastern Roman Empire ruling from Constantinople. While the Western Empire was overrun by Germanic barbarians the Eastern Empire thrived, and Constantinople became the largest city in the empire and a major commercial center. In 476 CE, the last Western Roman Emperor was deposed and the Western Roman Empire was no more. Its lands in Italy were conquered by the Ostrogoths, Spain was conquered by the Visigoths, North Africa was conquered by the Vandals, and Gaul was conquered by the Franks. This left the Eastern Roman Empire as the only Roman Empire standing.
"The Byzantine Empire: The Empire of New Rome" (Saylor) http://www.saylor.org/site/wp-content/uploads/2012/10/HIST101-11.1-ByzantineEmpire-FINAL1.pdf