This essay reveals the history of the restoration of the Roman Empire to its former borders under the emperors of the Justinian and Heraclid dynasties, when for the first time Byzantium had to wage “holy wars” with the Visigoths, Vandals and Persians. During this same period, its first and very tragic clashes with the emerging world of Islam occurred, which would accompany its entire subsequent history. This time was also characterized by fierce dogmatic disputes, for the resolution of which the Fifth, Sixth and Trullo (Fifth-Sixth) Ecumenical Councils were convened, saving the Catholic Church. The book is equipped with special appendices that provide background material on the ecclesiastical status of the Byzantine emperor, the evolution of the Roman army, as well as the emerging confrontation between the two great ecumenical ecclesiastical sees - Rome and Constantinople.