The Battle of Actium on September 2, 31 BC was not a particularly impressive naval engagement by the standards of ancient warfare. It was not especially close, not especially bloody, and not decided by brilliant tactics or unusual courage. What it was, was decisive in the way that very few battles actually are: it determined who would rule the Roman world for the next five centuries and set the conditions for everything that followed. Octavian defeated Mark Antony and Cleopatra at Actium, and Rome became an empire.

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.

Alexandria was not a Roman city. It was a Greek city under Roman administration, founded by Alexander the Great in 331 BC and designed from its inception as a world capital — a city that would connect the Mediterranean world to Egypt and, through Egypt, to the trade routes of the East. By the time Rome absorbed it as part of Egypt following Cleopatra’s death in 30 BC, Alexandria was already three centuries old, the second-largest city in the Mediterranean after Rome itself, and possessed of institutions — the Library, the Museum, the great lighthouse — that Rome had nothing to rival. The Romans did not conquer Alexandria so much as inherit it, and the inheritance was complicated.

Antioch on the Orontes — modern Antakya in southern Turkey near the Syrian border — was the third city of the Roman Empire and the capital of its eastern operations. After Rome and Alexandria, no city in the Mediterranean world was larger or more strategically important. It was the administrative center for the Syrian provinces, the supply base for Rome’s eastern wars, the commercial hub connecting the Mediterranean trade network to the silk and spice routes of Asia, and an early center of Christian organization so significant that the word Christian — Christianoi — was first used there. The city that matters to understanding Rome’s eastern empire is Antioch, and it is among the most underrepresented in the popular historical imagination.

Gaius Octavius was eighteen years old when Julius Caesar was assassinated in 44 BC, and nobody thought he mattered. He was Caesar’s great-nephew, slight and sickly, without military reputation or political standing. He had one asset: Caesar’s will named him adopted son and primary heir. He used that asset with a patience and calculation that none of his older, more experienced rivals understood until it was too late.

The name by which history knows him — Augustus, the revered one — was a title conferred by the Senate in 27 BC, seventeen years after Caesar’s death and four years after he had defeated Mark Antony and Cleopatra at the Battle of Actium, eliminating the last serious rival to his control of the Roman world. Between the teenager nobody feared and the man the Senate was now calling Augustus lay fourteen years of civil war, shifting alliances, calculated betrayals, and the systematic elimination of everyone who stood between Octavian and sole power. He was very good at it.

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.

Caesarea Maritima was built by Herod the Great on a site with no natural harbor, no fresh water source, and no existing urban infrastructure, over a period of approximately twelve years ending around 10 BC, and it became the capital of the Roman province of Judaea and one of the most important cities on the eastern Mediterranean coast. The feat of urban creation involved harbor engineering that modern marine archaeologists have called the most ambitious building project in the ancient world: an artificial harbor of roughly 100,000 square meters created by sinking enormous concrete blocks into water sixty meters deep, using hydraulic concrete — the pozzolanic technology that gave Roman harbor structures their extraordinary durability — in its most ambitious application anywhere in the empire. The harbor blocks, two thousand years later, still lie beneath the Mediterranean, structurally recognizable and studied by diving archaeologists who find in them evidence of Roman engineering at its most technically extraordinary.

On August 2, 216 BC, on a flat plain near the Aufidus River in southern Italy, the Carthaginian general Hannibal Barca destroyed a Roman army of approximately 86,000 men in a single afternoon. Somewhere between 47,000 and 70,000 Romans died — the numbers vary by ancient source but the scale is not in dispute. It was the bloodiest day in Roman history, possibly the bloodiest single day of battle in the ancient world, and it accomplished nothing. Rome did not fall. It did not negotiate. It raised more legions and kept fighting.

Carthage must be destroyed. The phrase — Carthago delenda est — was reportedly repeated by Cato the Elder at the end of every Senate speech he gave, regardless of the speech’s actual subject, in the years before the Third Punic War. It is probably apocryphal, or at least exaggerated, but it captures something true about the Roman relationship with Carthage: a fear and hostility so intense that it could only be resolved by elimination, and a political culture prepared to act on that resolution against a city that had been, by the 140s BC, effectively defanged by the terms of its previous defeat.

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.