Successors of the Isaurians and the Phrygian Dynasty (820-867)
External relations of the Byzantine Empire. Arabs and Slavs and the insurrection of Thomas the Slavonian. In the ninth century hostile relations between the Byzantine Empire and the Arabs were almost incessant. On the eastern land borderline these relations assumed the aspect of reiterated collisions which occurred with almost annual regularity and were accompanied by frequent exchanges of prisoners. On the Muhammedan side of the border a line of fortifications, intended as a defense against the attacks of the Byzantine army, was erected from Syria to the confines of Armenia. Similar fortified cities were to be found on the Byzantine side. All the fortifications formed a sort of limes in Asia Minor. Only in very few instances did the collisions along the eastern border in the ninth century assume the aspect of serious campaigns deep into the country. Parallel with the gradual political decline and weakening of the caliphate in the ninth century, which came as a result of serious internal disturbances and the predominance of Persians, and later of Turks, the continuous attacks of the Muslims upon the Byzantine Empire from the East ceased to threaten, as they did in the seventh and eighth centuries, the very existence of the Empire. These attacks continued, however, to bring great harm to the border provinces by injuring the prosperity of the population, by reducing their taxpaying ability, and by killing many of the inhabitants. The first thirty years of the ninth century were crowned by the reigns of the famous caliphs, Harun-ar-Rashid (786-809) and Mamun (813-33), under whom Persian influence enjoyed almost exclusive predominance and forced Arabian nationality into the background. In their political ideas the caliphs of the ninth century, particularly Mamun, resembled the Byzantine emperors in that they believed their authority to be unlimited in all phases of the life of their state.
Although the Arabo-Byzantine collisions in the East, with very few exceptions, did not result in any serious consequences for either side, the operations of the Muslim fleet in the Mediterranean Sea, which led to the occupation of Crete, the greater part of Sicily, and a number of important points in southern Italy, were of exceedingly great significance.