On August 9, 378 AD, the Eastern Roman emperor Valens led his army against a Gothic force near Adrianople in Thrace — modern Edirne in northwestern Turkey — and was killed along with roughly two-thirds of his army. The Battle of Adrianople was not the largest Roman defeat in history; Cannae killed more Romans in a single afternoon. It was not the most strategically complex engagement the Romans ever fought; the tactics were relatively straightforward. What made it consequential was not the battle itself but what came before it and what followed from it, the chain of decisions and consequences that makes Adrianople one of the pivots of late Roman history.

The Byzantine Empire called itself Rome. Its citizens called themselves Romans. Its emperor held the title that Augustus had held. Its laws were Roman laws. Its language of government was Latin until the seventh century, when Greek — which had always been the spoken language of the eastern provinces — became official. The entity that modern historians call Byzantium would not have recognized the name: it was Byzantium only in retrospect, named by scholars for the ancient Greek city on whose site Constantine had built his new capital. To everyone who lived in it, from Constantine’s founding of Constantinople in 330 AD to the Ottoman conquest in 1453, it was simply Rome. That it had relocated, that the western half had collapsed, that Germanic kings sat in Ravenna and eventually in the city of Rome itself — none of this changed the self-conception of an empire that understood itself as continuous with Augustus.

On Christmas Day, 800 AD, Pope Leo III placed a crown on the head of the Frankish king Charles in St. Peter’s Basilica in Rome and the assembled congregation acclaimed him Emperor of the Romans. Whether Charlemagne was surprised by this — his biographer Einhard claims he said he would not have entered the church had he known what was to happen — is debated; the staging suggests coordination, and Charlemagne was not a man who was often genuinely surprised by political events. What is not debated is what the coronation meant: four centuries after the conventional date of Rome’s fall, the most powerful ruler in western Europe was being crowned not as King of the Franks or King of the Germans but as Emperor of the Romans, in Rome, at the greatest shrine of Roman Christianity, by the successor of St. Peter. Rome had not died. It had changed form.

The kingdoms that replaced Roman administration in the western provinces were not anti-Roman. This is the most important correction to the standard narrative of Rome’s fall, and it matters because the standard narrative — civilized Rome overwhelmed by barbarous outsiders — is both factually wrong and interpretively misleading. The Visigothic kingdom in Spain, the Ostrogothic kingdom in Italy, the Burgundian kingdom in the Rhône valley, the Frankish kingdom in Gaul — these were not negations of Rome. They were, in varying degrees, continuations of Rome under different management, sustained by Roman administrative forms, legitimated by Roman imperial titles, and often governed by rulers who had spent significant portions of their careers in Roman service and who regarded Roman civilization as the culture they had inherited rather than the culture they had defeated.

On August 24, 410 AD, the Visigoths under Alaric entered Rome through the Salarian Gate and spent three days sacking the city. It was the first time a foreign enemy had taken Rome in eight hundred years — since the Gauls in 390 BC — and the psychological shock of the event reverberated across the Mediterranean world in ways that exceeded its military or economic significance. Jerome, writing from Bethlehem, described the impact in terms usually reserved for cosmic events. Augustine, prompted by pagan Romans who blamed Christianity for the calamity, spent the next thirteen years writing the City of God in partial response to the question of what the sack meant. What it meant, in fact, was both more and less than the commentary of the time suggested.

Edward Gibbon spent six volumes and most of his adult life explaining why Rome fell, and he was not the first. The question has generated more scholarly production than almost any other in historical study, a volume that says less about Roman history than about the intellectual needs of subsequent civilizations that measured themselves against Rome’s shadow. Every generation finds its own answer, and every answer reveals as much about the present as about the fifth century.