Its story
Iconoclasm ('image-breaking') was a Byzantine imperial policy against the veneration of icons. According to the traditional account, Emperor Leo III acted between 726 and 730 (removing an image of Christ from the palace's Chalke Gate and deposing Patriarch Germanus in 730). Modern historians (for example Brubaker and Haldon) caution that nearly all sources are later and were written by the victorious icon-supporters, so the extent of the first measures is uncertain.
Iconoclasts appealed to the Decalogue's prohibition of images and to some early writers. Under Constantine V, the synod of Hieria (754), with over 330 bishops, gave the position a Christological basis: an image of Christ either depicts only his humanity, separating it from his divinity (Nestorianism), or claims to depict the undepictable divinity and confuses it with flesh (Monophysitism); the only true image of Christ would be the Eucharist. The possible influence of Islam and Judaism, and the weight of political and military motives, are debated.
John of Damascus, a monk in Muslim-ruled Palestine and thus beyond imperial reach, wrote three treatises defending images: the invisible God became visible in the incarnation and can therefore be depicted; the honor paid to the image passes to its prototype (Basil's formula). The Second Council of Nicaea (787), convened by Empress Irene, restored icons and distinguished veneration (proskynēsis), due to images, from worship (latreia), due to God alone. In the West, a faulty translation of its acts prompted the reaction of the Libri Carolini and the Synod of Frankfurt (794), which rejected what they understood as 'adoration' of images.
Iconoclasm returned under Leo V in 815 and lasted until 843, when Empress Theodora restored the images; the Orthodox Church commemorates this every first Sunday of Lent as the 'Triumph of Orthodoxy'. Theodore the Studite and Patriarch Nicephorus were the chief defenders in this second phase. Later traditions differ in their assessment: Catholics and Orthodox accept Nicaea II as ecumenical; Protestant traditions do not treat it as binding and vary in their use of images, from Lutheranism, which kept them, to Reformed currents that removed them from worship.