The Eastern Roman or 'Byzantine' Empire had to fight for survival throughout its eleven centuries of history. Military ability was therefore a prime requisite for a successful Emperor. In Fighting Emperors of Byzantium, historian John Carr explores the personal and military histories of the fighters who occupied the imperial throne at Constantinople. They include men like its founder Constantine I , Julian, Theodosius, Justinian, Heraclius, Leo I, Leo III, Basil I, Basil II (the Bulgar-slayer), Romanus IV Diogenes, Isaac Angelus, and Constantine XI.
Byzantium's emperors, and the military establishment they oversaw, can be credited with preserving Rome's cultural legacy and, from the seventh century, forming a bulwark of Christendom against aggressive Islamic expansion. For this the empire's military organization had to be of a high order, a continuation of Roman discipline and skill adapted to new methods of warfare.