The Roman Empire outlasted the city of Rome by almost a thousand years. When the western half fell in the fifth century, the eastern half continued until 1453. Its capital was Constantinople, its language shifted from Latin to Greek, its institutions and self-understanding changed, and it lost territory steadily until almost nothing was left. Still, the line of emperors and the law they administered did not break. Its inhabitants called themselves Rhomaioi, Romans. “Byzantine” is a sixteenth-century Western coinage they would not have recognized.

Constantine founded Constantinople in 330, convened the Council of Nicaea in 325 to settle the question of Christ’s divinity, and was the first Roman emperor to rule as a Christian — funding churches, exempting clergy from taxation, and giving bishops legal authority. The eastern half of the empire was wealthier, more urbanized, and protected by the new capital’s land walls, while the western half was disintegrating under Germanic pressure. The east that survived was Roman in law and Christian in religion.

Justinian (527–565) was the last emperor who tried to govern the whole Mediterranean. He reconquered North Africa, much of Italy, and parts of southern Spain; codified Roman law into the Corpus Juris Civilis, which was rediscovered at Bologna in the eleventh century and became the foundation of civil-law systems across Europe and much of the world; and built Hagia Sophia. His reign also produced the Nika riots of 532, suppressed with heavy loss of life, and the arrival in 541 of the bubonic plague pandemic that bears his name, which returned in waves for two centuries. The Italian reconquest came apart within a generation as the Lombards took most of the peninsula. The treasury was exhausted, and none of his successors attempted anything on that scale again.

The seventh century brought the largest losses in the empire’s history. Heraclius (610–641) won a long war against Sasanian Persia by 628. Within a decade, Arab armies of the new Islamic caliphate took Syria in 636, Jerusalem in 638, and Egypt in 642, and destroyed the Sasanian state. What survived was rebuilt to fit the new circumstances. Cities contracted to defensible cores, the coin economy thinned, Greek replaced Latin in administration, and the provinces were reorganized into themes — military districts in which soldiers held land in exchange for service, producing a self-supporting army that did not require cash to maintain. The empire that emerged was smaller, poorer, more rural, and more religious, and it would last in that form for centuries.

From 726 to 843 the empire was divided by the iconoclast controversy. Emperors backed by parts of the church and army moved to suppress the veneration of images of Christ and the saints, and were resisted by monks, much of the population, and the popes in Rome. The argument turned on whether the divine, having taken on flesh in Christ, could be represented in matter, or whether such images collapsed the distance between creator and creation into idolatry. The defenders of images, led intellectually by John of Damascus and Theodore the Studite, prevailed in 843; the event is still commemorated in Orthodox churches as the Triumph of Orthodoxy. In the same century, the missionary brothers Cyril and Methodius brought Christianity to the Slavs and devised an alphabet for Slavic languages.

The Macedonian dynasty (late ninth to eleventh centuries) brought the empire to full strength within its new limits. Basil II (976–1025) destroyed the First Bulgarian Empire and pushed the frontier back to the Danube. Constantinople produced a literary revival, and most surviving ancient Greek literature reaches us through manuscripts copied in this period in a new compact script. In 1054 the patriarch of Constantinople and a papal legate excommunicated each other; contemporaries treated this as one quarrel among many, and only much later was it read as the formal schism between the Orthodox and Catholic churches.

In 1071 the Seljuk Turks defeated the Byzantine army at Manzikert. The battle itself was not catastrophic, but the civil war that followed left the interior undefended, and Turkish groups settled across most of Anatolia, which had been the empire’s main recruiting ground and tax base. The Komnenian emperors (1081–1180) rebuilt on a smaller footing, replacing the theme-based army with foreign mercenaries and paying for them partly by granting trade privileges to Venice and Genoa. The arrangement worked for a century, but it embedded Italian commercial interests inside the imperial economy. In 1204 the Fourth Crusade, diverted by Venetian pressure and a Byzantine succession dispute, sacked Constantinople and installed a Latin emperor in the city.

Michael VIII Palaiologos recaptured Constantinople in 1261, but what he recovered was closer to a city-state with outlying territories than an empire. Its remaining history was one of civil wars, recurring plague, and steady territorial losses to the rising Ottoman emirate. It also produced one of the strongest intellectual periods in its history: Gregory Palamas refined Orthodox theology by distinguishing between God’s unknowable essence and his uncreated energies, through which God is genuinely encountered, and Plethon lectured on Plato in Florence in 1438–39 and influenced Italian humanists. A church union with Rome negotiated at the Council of Florence that same year in exchange for Western military aid was rejected by the Orthodox population at home, and sufficient aid never came.

Constantinople fell to Mehmed II in 1453 after a fifty-three-day siege. The Ottomans took the city using cannon — including a large gun cast by a Hungarian engineer named Orban, who had offered his services to the Byzantines first and gone to the Ottomans when the imperial treasury could not match their price — and a celebrated maneuver in which ships were dragged overland on greased logs to bypass the harbor chain. The last emperor, Constantine XI Palaiologos, died in the fighting. The state had existed in some form for 1,123 years.