Byzantine Empire
The Greek-speaking eastern half of Rome that outlived the west by a thousand years, from Constantinople's founding in 330 to its fall in 1453.
The Roman Empire did not fall in 476. Its richer, Greek-speaking eastern half sailed on for almost another thousand years, and the people who lived in it never once called themselves Byzantine. To them the label would have been nonsense: they were Romaioi, Romans, subjects of an empire running in unbroken line from Augustus. The word Byzantine was coined by scholars long after the empire was gone, borrowed from Byzantion, the old Greek town that Emperor Constantine refounded in 330 as his New Rome on the Bosphorus and modestly renamed Constantinople.
New Rome
For centuries this was the richest, most sophisticated state in the Christian world, and a wall against the East. While western Europe splintered into small kingdoms, Constantinople stood as a fortified metropolis of half a million people, its harbors thick with trade and its triple land walls unbroken for the better part of eight centuries. It absorbed the shock of Persian, Arab, and Turkish armies that might otherwise have rolled into a defenseless West, which is why it is often called the shield of Europe. Its emperor Justinian, in the sixth century, ordered the vast codification of Roman law known as the Corpus Juris Civilis, still the root of most European legal systems, and built Hagia Sophia, a domed church so enormous that legend says he cried out that he had outdone Solomon. Byzantine engineers guarded a secret weapon, Greek fire, a liquid that burned on water and incinerated enemy fleets.
Two Romes, then one loss
Faith slowly split the old empire in two. Centuries of dispute over doctrine, ritual, and the authority of the pope hardened into the Great Schism of 1054, the permanent break between the Roman Catholic West and the Greek Orthodox East that still divides Christianity. The rupture turned lethal in 1204, when the Fourth Crusade, an army raised to fight Muslims, was diverted to Constantinople instead, stormed the Christian city, and looted it so thoroughly that the bronze horses of its hippodrome still stand over the doorway of St Mark's in Venice. The empire that limped out of that catastrophe was a shadow, a handful of provinces around a capital far too big for it.
The scholars who fled
By the fifteenth century Byzantium was a city-state pretending to be an empire, and in 1453 Mehmed II's cannon finished it. Yet its long afterglow shaped the modern world. For a thousand years its monks had copied and preserved the classical Greek texts, Plato, Aristotle, Euclid, that the Latin West had largely lost. As the Ottomans closed in, Greek scholars carried those manuscripts to Italy, where they helped ignite the Renaissance rediscovery of the ancient world. The empire also handed its faith and its imperial idea north to the Slavs: Russia took Orthodox Christianity, the Cyrillic alphabet, and eventually the claim that Moscow was the Third Rome, heir to a city that had already fallen twice. The Romans of the East vanished in 1453, but their law, their alphabet, their church, and the books they refused to lose outlived them by a wide margin.