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 unconquered sun — Sol Invictus — was the dominant religious force in the Roman Empire during the decades immediately before Christianity became the state religion, and the competition between them was closer than the outcome suggests. Aurelian, who reunified the empire after the chaos of the third century and who is one of the more underrated figures in Roman imperial history, established Sol Invictus as the supreme deity of the Roman state in 274 AD, built a spectacular temple in Rome, and created a new priesthood — the pontifices Solis — to administer its cult. For roughly forty years, the sun god was in a position of official supremacy that Christianity would not achieve until the reign of Theodosius nearly a century later.