Leo III the Isaurian (Greek Λέων ο Ίσαυρος) (c. 680–June 18, 741) was Byzantine emperor from 717 until his death. He put an end to a period of instability, successfully defended the empire against the invading Arabs, and adopted the religious policy of iconoclasm.
Leo was born in the Syrian province of Commagene. He rose to distinction in the military, and under Anastasius II he was given the command of the eastern army. In 717, he revolted against the usurper Theodosius III and, marching upon Constantinople, was elected emperor in his place. The first year of Leo's reign saw an important siege of his capital by the Arabs, who had taken advantage of the civil discord in the Byzantine empire to bring a force of 80,000 men to the Bosporus. By his stubborn defence the new ruler wore out the invaders who, after a twelve months' siege, withdrew their forces. An important factor in the victory of the Byzantines was their use of Greek fire. Having thus preserved the empire from extinction, Leo proceeded to consolidate its administration, which in the previous years of anarchy had become completely disorganized. He secured its frontiers by inviting Slavic settlers into the depopulated districts and by restoring the army to efficiency; when the Arabs renewed their invasions in 726 and 739 they were decisively beaten. His civil reforms include the abolition of the system of prepaying taxes which had weighed heavily upon the wealthier proprietors, the elevation of the serfs into a class of free tenants and the remodelling of family and of maritime law. These measures, which were embodied in a new code published in 740, met with some opposition on the part of the nobles and higher clergy.
But Leo's most striking legislative reforms dealt with religious matters, especially iconoclasm. After an apparently successful attempt to enforce the baptism of all Jews and Montanists in the empire (722), he issued a series of edicts against the worship of images (726-729). This prohibition of a custom which had undoubtedly given rise to grave abuses seems to have been inspired by a genuine desire to improve public morality, and received the support of the official aristocracy and a section of the clergy. But a majority of the theologians and all the monks opposed these measures with uncompromising hostility, and in the western parts of the empire the people refused to obey the edict. A revolt which broke out in Greece, mainly on religious grounds, was crushed by the imperial fleet (727), and two years later, by deposing the patriarch of Constantinople, Leo suppressed the overt opposition of the capital. In Italy the defiant attitude of Popes Gregory II and Gregory III on behalf of image-veneration led to a fierce quarrel with the emperor. The former summoned councils in Rome to anathematize and excommunicate the iconoclasts (730, 732); Leo retaliated by transferring southern Italy and Greece from the papal diocese to that of the patriarch. The struggle was accompanied by an armed outbreak in the exarchate of Ravenna (727), which Leo finally endeavoured to subdue by means of a large fleet. But the destruction of the armament by a storm decided the issue against him; his south Italian subjects successfully defied his religious edicts, and the province of Ravenna became detached from the empire.