Literally, Iconoclasm is religious and political destruction of the sacred images or monuments, usually (though not always) of another religious group. People who destroy such images are called iconoclasts, while people who revere or venerate such images are called iconodules.
The term "iconoclast" has come to have a more general meaning.
Emperor Leo III the Isaurian (reigned 717-741) banned the use of icons of Jesus, Mary, and the Saints and commanded the destruction of these images in 730. The Iconoclastic Controversy was fueled by the refusal of many Christians resident outside the Byzantine Empire, including many Christians living in the Islamic Caliphate to accept the emperor's theological arguments. St. John of Damascus was one of the most prominent of these. Ironically, Christians living under Muslim rule at this time had more freedom to write in defense of icons than did those living in the Byzantine Empire. Leo was able to promulgate his policy because of his personal popularity and military success - he was credited with saving Constantinople from an Arab siege in 717-718 and then sustaining the Empire through annual warfare.
Leo III's son, Constantine V (reigned 741-775) was challenged at once by a general who used Iconophile ("Icon-favoring") propaganda, but his military success against this threat cemented his own position.
The first Iconoclastic period came to an end at the Second Council of Nicaea in 787, when the veneration of icons was affirmed, although the worship of icons was expressly forbidden. Among the reasons were the doctrine of the Incarnation: because God the Son (Jesus Christ) took on flesh, having a physical appearance, it is now possible to use physical matter to depict God the Son, and to depict the saints. Icon veneration lasted through the reign of Empress Irene's successor, Nicephorus I (reigned 802-811), and the two brief reigns after his.
Emperor Leo V (reigned 813-820) instituted a second period of Iconoclasm in 813, which seems to have been less rigorously enforced, since there were fewer martyrdoms and public destructions of icons. Leo was succeeded by Michael II, who was succeeded by his son, Theophilus II. Theophilus died leaving his wife Theodora regent for his minor heir, Michael III. Like Irene 50 years before her, Theodora mobilized the iconodules and proclaimed the restoration of icons in 843. Since that time the first Sunday of Lent is celebrated in the churches of the Orthodox tradition as the feast of the "Triumph of Orthodoxy".
Because of the prohibition against figural decoration in mosques - not, as is often said, a total ban on the use of images - Muslims have on occasion committed acts of iconoclasm against the devotional images of other religions. An example of this is the 2001 destruction of frescoes and the monumental statues of the Buddha at Bamiyan by the Taliban, an element of the Islamist movement.
In a number of countries, conquering Muslim armies tore down local temples and houses of worship, and built mosques on their sites. The Dome of the Rock in Jerusalem was built on top of the remains of the Jewish Temple in Jerusalem. Similar acts occurred in parts of north Africa under Muslim conquest. In India, numerous former Buddhist monasteries and Hindu temples were conquered and rebuilt as mosques. In recent years, some Hindu nationalists have attempted to tear down these mosques, and replace them with Hindu Temples. This is part of the current conflict today between Indian Hindu nationalists and Indian Islamists.
Some of the Protestant reformers encouraged their followers to destroy Catholic art works by insisting that they were idols. Huldreich Zwingli and John Calvin promoted this approach to the adaptation of earlier buildings for Protestant worship. In 1562, some Calvinists destroyed the tomb of St. Irenaeus and the relics inside, which had been under the altar of a church since his martyrdom in 202.
The Netherlands (including Belgium) were hit by a large wave of Protestant iconoclasm in 1566. This is called the Beeldenstorm.
Bishop Joseph Hall of Norwich described the events of 1643 when troops and citizens, encouraged by a Parliamentary ordinance against superstition and idolatry, behaved thus:
Byzantine Iconoclasm
The First Iconoclastic Period: 730-787
The Second Iconoclastic Period: 813-843
Islamic Iconoclasm
Reformation Iconoclasm