The contraction of the Christian faith and Christian societies in Asia and Africa left Europe and Anatolia, largely by default, as the centers of Christendom. The initial expansion of Islam and the Arab Empire had quickly stripped away what had been the Middle Eastern and North African provinces of the Roman Empire and had brought Spain under Muslim control. But after the Mediterranean frontier between the Islamic and Christian worlds stabilized somewhat in the early eighth century, the immediate threat of Muslim incursions into the heartland of Christendom lifted, although border conflicts persisted. It was within this space of relative security, unavailable to most African and Asian Christian communities, that the diverging histories of the Byzantine Empire and Western Europe took shape.

Unlike most empires, Byzantium has no clear starting point. Its own leaders, as well as its neighbors and enemies, viewed it as simply a continuation of the Roman Empire. Some historians date its beginning to 330 C.E., when the Roman emperor Constantine, who began to favor Christianity during his reign, established a new capital, Constantinople, on the site of an ancient Greek city called Byzantium. At the end of that century, the Roman Empire was formally divided into eastern and western halves, thus launching a division of Christendom that has lasted into the twenty-first century.

Although the western Roman Empire collapsed during the fifth century, the eastern half persisted for another thousand years. Housing the ancient civilizations of Egypt, Greece, Syria, and Anatolia, the eastern Roman Empire (Byzantium) was far wealthier, more urbanized, and more cosmopolitan than its western counterpart; it possessed a much more defensible capital in the heavily walled city of Constantinople; and it had a shorter frontier to guard. Byzantium also enjoyed access to the Black Sea and command of the eastern Mediterranean. With a stronger army, navy, and merchant marine as well as clever diplomacy, its leaders were able to deflect the Germanic and Hun invaders who had overwhelmed the western Roman Empire.

Much that was late Roman—its roads, taxation system, military structures, centralized administration, imperial court, laws, Christian Church—persisted in the east for many centuries. Like Tang dynasty China seeking to restore the glory of the Han era, Byzantium consciously sought to preserve the legacy of classical Greco-Roman civilization. Constantinople was to be a “New Rome,” and people referred to themselves as “Romans.” Fearing contamination by “barbarian” customs, emperors forbade the residents of Constantinople from wearing boots, trousers, clothing made from animal skins, and long hairstyles, all of which were associated with Germanic peoples, and insisted instead on Roman-style robes and sandals. But much changed as well over the centuries, marking the Byzantine Empire as the home of a distinctive civilization.