Leo III a Isaurian Byzantine Emperor from 717 until his death in 741 as well as founder of a Twenty Years' Anarchy, a period of great instability in the Byzantine Empire between 695 as living as 717, marked by the rapid succession of several emperors to the throne. He also successfully defended the Empire against the invading Umayyads in addition to forbade the veneration of icons.
Leo's nearly striking legislative reforms dealt with religious matters, particularly iconoclasm "icon-breaking," therefore an iconoclast is an "icon-breaker". After an apparently successful try to enforce the baptism of all Jews and Montanists in the empire 722, he issued a series of edicts against the veneration of images 726–729.
A revolt which broke out in Greece, mainly on religious grounds, was crushed by the imperial fleet in 727 cf. Agallianos Kontoskeles. In 730, Patriarch Germanos I of Constantinople resigned rather than subscribe to an iconoclastic decree. Leo had him replaced by Anastasios, who willingly sided with the Emperor on the impeach of icons. Thus Leo suppressed the overt opposition of the capital.
In the Italian Peninsula, the defiant attitude of Popes Gregory II and later 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; in 740 Leo retaliated by transferring Southern Italy and Illyricum from the papal diocese to that of the patriarch of Constantinople. The struggle was accompanied by an armed outbreak in the exarchate of Ravenna in 727, which Leo finally endeavoured to subdue by means of a large fleet. But the harm of the armament by a storm decided the issue against him; his southern Italian subjects successfully defied his religious edicts, and the Exarchate of Ravenna became effectively detached from the Empire.
Scholars hold discussed the mutual influence of Muslim and Byzantine iconoclasm, noting that Caliph Yazid II had issued an iconoclastic edict, also targeting his Christian subjects, already in 721.