In April 491, a crowd in the Hippodrome of Constantinople demanded an emperor who was Orthodox and Roman. Six years later, the severed heads of two rebel leaders stood on poles above the Golden Horn.

Between those moments lies one of the most decisive and least known conflicts of the late Roman world. For a generation, the Isaurians, a mountain people of the Taurus long dismissed as brigands, had ruled the Eastern Empire, guarding its palace, commanding its armies, and placing one of their own, Zeno, on the throne. When he died, his widow Ariadne passed over his brother and chose an ageing palace official named Anastasius. The Isaurians expected a caretaker. They got the most determined administrator ever to wear the purple.

Drawing on fragmentary chronicles and the archaeology of the Taurus, Marcus Behan reconstructs the expulsion of the Isaurian establishment, the battle of Cotyaeum, the siege of Claudiopolis, the slow starvation of the mountain fortresses, and the triumph in the Hippodrome. He argues that this was no frontier police action but a Roman civil war, one that rescued the East from the fate of the West and built the treasury and army on which Justinian's conquests rested.

This is the story of how an empire chose what it would become.