The eastern Roman Empire centered on Constantinople until its fall in 1453.
The Byzantine Empire was Rome continued: the eastern empire ruling from Constantine's city for over a millennium (330–1453), Greek in tongue, Roman in law and title, Orthodox in faith — Justinian's codification and Hagia Sophia its early summits, the theme-system its medieval survival kit.
Its medieval arc swings vast: Heraclius's Persian victory devoured by the Arab conquests; iconoclasm's century; the Macedonian apogee under Basil II; the Battle of Manzikert (1071) opening Anatolia to the Turks; Komnenian restoration entangled with the crusades it summoned; the Fourth Crusade's sack (1204) and Palaiologan recovery (1261) of a shrinking realm — until Mehmed II's guns ended it at the Fall of Constantinople, 29 May 1453.
Constantine founded the city (330) and Theodosius's heirs split the administration for good (395); the east, richer and walled, survived the west's fall and under Justinian (527–565) counter-attacked — Belisarius's Africa and Italy, the Corpus Juris Civilis, Hagia Sophia's dome — before plague and Persian wars spent the surplus.
The seventh century remade it or nothing would remain: Heraclius destroyed Persia (Nineveh, 627) only to lose Syria, Egypt, and Africa to the Arab caliphate within a generation; Constantinople itself held the sieges of 674–678 and 717–718 — Greek fire and the Theodosian walls saving, arguably, Europe. The response was systemic: themes settling soldier-farmers under strategoi, Greek replacing Latin, the empire hardening into its medieval form — smaller, militarised, and indestructible for centuries.
The iconoclast controversy (726–787, 815–843) fused theology and politics — emperors against images, monks and empresses for them — ending with Orthodoxy's "Triumph" (843) and a self-definition (icons, liturgy, Roman order) exported wholesale: Cyril and Methodius to the Slavs (863), Bulgaria converted (864), and in 988 Vladimir of Kiev's baptism marrying Rus' to Byzantium — the "commonwealth" of Orthodox nations born.
The Macedonian dynasty (867–1056) rode reconquest to apogee: Nikephoros Phokas and John Tzimiskes retaking Crete, Cilicia, Antioch; Basil II (976–1025) grinding Bulgaria to annexation (1018, "Bulgar-slayer") and leaving the treasury full, the frontiers at the Danube and Euphrates — and the succession fatally unarranged: fifty years of court mismanagement squandered it into Manzikert's decade.
Manzikert (1071) and the civil wars after let the Seljuks settle Anatolia — the recruiting heartland — while Normans took Italy's last themes (Bari, 1071). Alexios I Komnenos rebuilt through alliance: his appeal helped summon the First Crusade, whose passage recovered Nicaea and the coasts but planted Latin states with Byzantine claims — a century of Komnenian management (John II's campaigns, Manuel's ambitions) ending at Myriokephalon (1176) and, after the dynasty's fall, in catastrophe: the Fourth Crusade, diverted by Venice and exiled princes, stormed and sacked Constantinople (12–13 April 1204), partitioning the empire into Latin fiefs.
The Greek successor-states (Nicaea, Epirus, Trebizond) kept the flame; Michael VIII Palaiologos retook the city (1261) and refounded an empire that was thereafter a great name defending shrinking ground: Catalan companies and civil wars, the Ottoman crossing at Gallipoli (1354), Bayezid's blockade broken only by Timur at Ankara (1402), union offered at Florence (1439) for help that died at Varna (1444) — until Mehmed II's two-month siege ended it: Constantine XI Palaiologos falling at the walls on 29 May 1453, the city becoming an Ottoman capital, and "Rome" — after 2,206 years by its own count — passing into memory and claim.
Byzantium was medieval Europe's only bureaucratic state: an emperor "crowned by God" (and made by army, senate, and mob in practice — dynasties changed by acclamation and blinding), a salaried civil service of logothetes and themes' governors, Roman law continuously updated (Justinian's Corpus, the Ecloga, Basil I's Basilika), gold coinage (the nomisma) stable for seven centuries, and a diplomatic service whose bribes, marriages, and ceremony (the Book of Ceremonies codifying awe) were weapons as real as the tagmata.
Church and empire ran as symphonia: the patriarch crowning, the emperor convening councils and appointing — caesaropapism to critics, harmony to theorists — with monasticism (Athos chartered 963) as the spiritual counterweight. The system's genius was reproduction: every collapse (7th century, 1204) produced administrations-in-exile that reassembled the state from its paperwork.
Justinian I, 527–565 — the law codified, the west briefly retaken, Hagia Sophia raised.
Heraclius, 610–641 — Persia destroyed, the Cross restored, the Arab storm endured.
Leo III, 717–741 — the 717 siege broken; iconoclasm begun.
Basil II, 976–1025 — the Bulgar-slayer's apogee: Danube to Euphrates.
Romanos IV Diogenes, 1068–1071 — Manzikert's captive emperor.
Alexios I Komnenos, 1081–1118 — the restorer who called the West and managed what answered.
John II Komnenos, 1118–1143 — the dynasty's best soldier-administrator.
Michael VIII Palaiologos, 1259–1282 — 1261's recoverer; union's first gambler.
John VIII Palaiologos, 1425–1448 — Florence's union for Varna's crusade.
Constantine XI Palaiologos, 1449–1453 — the last emperor, dead in the breach.