History isn’t supposed to hinge on the actions of a single man. Vast impersonal forces are supposed to sweep humanity along on an irresistible tide without regard to individual lives. But on a crisp fall day in AD 324, history hung on the shoulders of a man named Constantine as he climbed up a hill overlooking the Bosporus. Striding confidently forward, spear firmly in hand, he led a solemn procession of astonished courtiers. He had come following a divine voice—although whether it was that of an angel or of God himself he didn’t say. The turmoil of the recent civil wars was at last over. Once again the world lay at rest beneath the wings of the Roman eagle, but Rome itself, with its malarial streets and pagan past, was no longer worthy to be the capital of the world. So the young emperor had gone to Troy, that fabled cradle of the Roman people, and started work on a new capital. It was there, in the shadow of the ruined Trojan gates, that the voice first came to him. Priam’s ancient city, it said, was a city of the past, and so it should remain. His destiny—and that of his empire—lay elsewhere. Over the Hellespont it beckoned him, and he followed to the thousand-year-old city of Byzantium. That night he dreamed of an old woman who suddenly became young again, and when he awoke, he knew that on this spot he would make his capital. Rome, old and decrepit, would, like the woman in his dreams, be refreshed here on the shores of the Propontis.

So at least runs the legend, and the empire centered on Constantine’s New Rome would indeed grow vibrant once again. Refounded on a new, eastern, Christian axis, it would last for over a millennium, a shining beacon of light in a dark and turbulent world. Looking back, historians would claim that so much had changed in the moment of the city’s founding that the Roman Empire itself had been transformed into something else, and Byzantine history had begun.

But the roots of this new world didn’t begin with Constantine. The empire that he seized control of in the first decades of the fourth century had been profoundly changing for a generation, both politically and religiously, and Constantine merely put the finishing touches on its transformation. His vision and energy may have built the impressive edifice of Constantinople, but the reforms of his predecessor, Diocletian, provided the brick and mortar. And it is with Diocletian that the story of Byzantium properly begins.

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