While Constantine Porphyrogenitus had been dragging out the monotonous years of his long reign, events which completely changed the aspect of affairs in the Moslem East had been following each other in quick succession on the Asiatic frontier of his realm. Ever since it first came into existence the Byzantine Empire had been faced in Asia by a single powerful enemy; first by the Sassanian kingdom of Persia, then by the Caliphate under the two dynasties of the Ommeyades and the Abbasides. Now, however, the Caliphate had at last broken up, and the descendants of Abdallah-es-Saffah and Haroun-al-Raschid had become the vassals of a rebellious subject, and preserved a mere nominal sovereignty which did not extend beyond the walls of their palace in Bagdad. The crisis had come in 951 A.D., when the armies of the Buhawid prince Imad-ud-din, who had seized on the sovereignty of Persia, broke into Bagdad and made the Caliph a prisoner in his own royal residence. For the future the Caliphs were no more than puppets, and the Buhawid rulers used their names as a mere form and pretence. But the conquerors did not gain possession of the whole of the Caliphate; only Persia and the Lower Euphrates Valley obeyed them. Other dynasties rose and fought for the more western provinces of the old Moslem realm. The Emirs of Aleppo and Mosul, who ruled respectively in North Syria and in Mesopotamia, became the immediate neighbours of the East-Roman Empire, while the lands beyond them, Egypt and South Syria, formed the dominions of the house of the Ikshides. Thus the Byzantines found on their eastern frontier no longer one great centralized power, but the comparatively weak Emirates of Aleppo and Mosul, with the Buhawid and Ikshidite kingdoms in their rear. The four Moslem states were all new and precarious creations of the sword, and were generally at war with each other. An unparalleled opportunity had arrived for the empire to take its revenge on its ancient enemies and to move back the Mahometan boundaries from the line along the Taurus where they had so long been fixed. Fortunately it was not only the hour that had arrived, but also the man. The empire had at its disposal at this moment the best soldier that it had possessed since the death of Leo the Isaurian. Nicephorus Phocas was the head of one of those great landholding families of Asia Minor who formed the flower of the Byzantine aristocracy; he owned broad lands in Cappadocia, along the Mahometan frontier. His father and grandfather before him had been distinguished officers, for the whole race lived by the sword, but Nicephorus far surpassed them. He was not only a practical soldier, but a military author: his book, Περὶ Παραδρόμης πολέμου, dealing with the organization of armies, still survives to testify to his capacity.