The Eastern Roman Empire, also known as the Byzantine Empire, was forced to fight in order to survive, which is why military skill was a basic requirement for a successful emperor. John Carr focuses on the personal and military stories of the most capable warriors who attempted to ascend to the imperial throne of Constantinople. Among them are its founder, Constantine I, Julian, Theodosius, Justinian, Heraclius, Leo I, Leo III, Basil I, Basil II the Bulgar-Slayer, Romanos IV Diogenes, Isaac Angelos, and Constantine Palaiologos.

The emperors of Byzantium and the military regime they formed are credited with preserving the cultural heritage of Rome and, from the seventh century onwards, creating a bulwark of Christianity against the aggressive spread of Islam. For this reason, the military organization of the empire had to be of a high standard: maintaining Roman discipline and skill, adapted to new military tactics. Thus, under the leadership of its warrior emperors, the empire managed to survive for almost a thousand years after the fall of Rome.