A New Rome

The term "Byzantine" derives from Byzantium, an ancient Greek colony founded by a man named Byzas. Located on the European side of the Bosporus (the strait linking the Black Sea to the Mediterranean), the site of Byzantium was ideally located to serve as a transit and trade point between Europe and Asia Minor. In 330 A.D., Roman Emperor Constantine I chose Byzantium as the site of a new Roman capital, Constantinople. Five years earlier, at the Council of Nicaea, Constantine had established Christianity (once an obscure Jewish sect) as Rome's official religion. The citizens of Constantinople and the rest of the Eastern Roman Empire identified strongly as Romans and Christians, though many of them spoke Greek and not Latin.

Though Constantine ruled over a unified Roman Empire, this unity proved illusory after his death in 337. In 364, Emperor Valentinian I again divided the empire into western and eastern sections, putting himself in power in the west and his brother Valens in the east. The fate of the two regions diverged greatly over the next several centuries. In the west, constant attacks from German invaders such as the Visigoths broke the struggling empire down piece by piece until Italy was the only territory left under Roman control. In 476, the barbarian Odoacer overthrew the last Roman emperor, Romulus Augustus, and Rome had fallen.

Survival of the Byzantine Empire

The eastern half of the Roman Empire proved less vulnerable to external attack, thanks in part to its geographic location. With Constantinople located on a strait, it was extremely difficult to breach the capital's defenses; in addition, the eastern empire had a much shorter common frontier with Europe. It also benefited greatly from a stronger administrative center and internal political stability, as well as great wealth compared with other states of the early medieval period. The eastern emperors were able to exert more control over the empire's economic resources and more effectively muster sufficient manpower to combat invasion. As a result of these advantages, the Eastern Roman Empire–variously known as the Byzantine Empire or Byzantium–was able to survive for centuries after the fall of Rome. 

Though Byzantium was ruled by Roman law and Roman political institutions, and its official language was Latin, Greek was also widely spoken, and students received education in Greek history, literature and culture. In terms of religion, the Council of Chalcedon in 451 officially established the division of the Christian world into five patriarchates, each ruled by a patriarch: Rome (where the patriarch would later call himself pope), Constantinople, Alexandria, Antioch and Jerusalem. The Byzantine emperor was the patriarch of Constantinople, and the head of both church and state. (After the Islamic empire absorbed Alexandria, Antioch and Jerusalem in the seventh century, the Byzantine emperor would become the spiritual leader of most eastern Christians.)

The Byzantine Empire Under Justinian

Justinian I, who took power in 527 and would rule until his death in 565, was the first great ruler of the Byzantine Empire. During the years of his reign, the empire included most of the land surrounding the Mediterranean Sea, as Justinian's armies conquered part of the former Western Roman Empire, including North Africa. Many great monuments of the empire would be built under Justinian, including the domed Church of Holy Wisdom, or Hagia Sophia (532-37 A.D.). Justinian also reformed and codified Roman law, establishing a Byzantine legal code that would endure for centuries and help shape the modern concept of the state.

At the time of Justinian's death, the Byzantine Empire reigned supreme as the largest and most powerful state in Europe. Debts incurred through war had left the empire in dire financial straits, however, and his successors were forced to heavily tax Byzantine citizens in order to keep the empire afloat. In addition, the imperial army was stretched too thin, and would struggle in vain to maintain the territory conquered during Justinian's rule. During the seventh and eighth centuries, attacks by Persians and Slavs, combined with internal political instability and economic regression, threatened the empire. A new, even more serious threat arose in the form of Islam, founded by the prophet Muhammad in Mecca in 622. In 634, Muslim armies began their assault on the Byzantine Empire by storming into Syria. By the end of the century, Byzantium would lose Syria, the Holy Land, Egypt and North Africa (among other territories) to Islamic forces.