Iconoclasm was an 8th-9th century Byzantine heresy that briefly created a deep rift between the Church and the imperial government.

Dogma

The iconoclasts believed that the worship of icons (Greek εικων, image) was sinful--hence the name, which means "icon-breakers".

Icons have great significance for Orthodox practice, perhaps even more so than for the Catholic. This is plainly visible; in any Orthodox church, the iconostasis--a wall or screen of some kind consisting of or decorated with icons--shields the altar and draws all eyes. The walls and ceiling of the church are also covered with icons drawn with various techniques--in many churches, iconic frescoes dating back hundreds of years are still to be seen. There are icons in the majority of Orthodox homes. . Needless to say, icons are the Orthodox world's chief form of religious art, and many constitute cultural treasures.

It is commonly assumed that, much like the Muslims, iconoclasts were against icons because they were idolatrous. This is only partially true; this argument was used during the heresy's initial period. It was quickly abandoned, however, as it became apparent that the argument didn't hold water. Idols are images necessarily of deities; it is thought that they have power in and of themselves. This cannot apply to Christianity, because Christ is both completely God and completely man, and an icon is an image of Christ the man. Many or most icons are not even of Christ; they depict the saints, and are, for one thing, a good way to educate the flock about them, and a way of recognizing sainthood. The icon in itself means nothing, but the icon as a religious symbol means a lot of things.1 Generally, by the latter stage of iconoclasm (the period 784-847, where it reached its peak), there was little argumentation to speak of: the emperors--bent on subduing the rebellious clerics--did not argue with words, but with swords and torches. A common argument of the more rational iconoclasts, though, concerned the great (perceived) excess of ritual in contemporary Byzantine society, which was seen to be replacing real faith. Interestingly, the Cross was still very much venerated by the iconoclasts. "They have removed the holy cross from the churches and replaced it with images," lamented one.

History

Sergius-John VI (610-714)

We worship only one will in Christ, for God has perceived
not our sin, but our substance, as it was before the Fall.

-from letter of Pope Honorius to Patriarch Sergius, 634

The first part of this period was marked with another heresy (or pair thereof): monophysitism/monothelitism. The orthodox doctrine stated that there was one Christ who consisted of two distinct natures, and was both human and divine. Essentially, the monophysites believed that Jesus Christ was of only one nature, both divine and human. The monothelites said that Christ's will was also one. To a layman, especially a modern one, it is difficult to understand the distinction between these views and the orthodox position, especially considering that Nestorianism, the doctrine of there being two separate natures of Christ, was also a heresy! Monophysitism, incidentally, began as an opposition to Nestorianism, but went a bit too far. The distinction between these three teachings lies in the number of hypostases and natures that they held Christ to have.



Nestorian	    orthodox		  Monophysite
 2		      2			   1	  Natures
 2		      1			   1	  Hypostases
 1		      1			   1	  Persons


The 'hypostasis' is a sort of underlying substance underneath the Natures of Christ. So, the Nestorians thought that Christ had two of them, as well as two natures, the orthodox held that Christ had one hypostasis and two natures, and the monophysites that Christ had one person, one hypostasis, and one nature.

In any case, the patriarch of Byzantium fell into this heresy (or, rather, monothelitism, which would add a single will as well as a single nature to the mix). Except it wasn't one patriarch, or even two. Four patriarchs were anathematized by the VI Ecumenical Council in Constantinople--Sergius, Peter, Paul II, and Pyrrhos. This severely damaged the authority and prestige of the patriarchate; the next three patriarchs met fates of varying degrees of misfortune. Patriarch St. Callinicus was blinded, then buried alive, while John VI had to go crying to the Pope when the emperor that was his benefactor was overthrown. This weakness set the stage for the first period of iconoclasm, driven by emperor Leo III.

Germanus-Paul IV (715-784 (787))

Hearken to us, emperor: abandon your present course
and accept the holy church as you found her, for matters of
faith and practice concern not the emperor, but the pope.

- from letter of Pope Gregory III to Emperor Leo III

Emperor Leo III the Isaurian was a warrior. He was a crusader; his religious reforms were as far-reaching as his campaigns. He persecuted and mandatorily-baptized the Jews and numerous smaller sects. He fought major battles with the Arabs, mostly successfully. He passed a code of laws that rivaled Justinian's in significance. He was also an iconoclast.

It is not clear how he came to be an iconoclast. Though this appears strange, there is some evidence (two letters of indeterminate authenticity) that the Arab khalif Omar II tried to convert him to Islam, and was not successful, except for instilling in him the hatred of images. It is said that he was also a rationalist who was put off by image-worshipping in general. In any case, he seized upon the idea and began to enforce it. The patriarch St. Germanus, who is revered as a sort of saintly and steadfast defender of icon worship, put up a rather meek defense, preferring to respond to less highly-placed theological opponents, such as Thomas of Claudiopolis. Germanus has a mythos associated with him; it is now clear that much of the legendary aura surrounding his defense of orthodoxy is due to later propaganda, which exploited his image for political purposes.

The period 726-730 marked a surge in Leo's repression of orthodoxy. In 726, Leo replaced the icon of Christos Chalkites, which sat above the gate of the Great Imperial Palace, with a simple cross and a self-adulatory inscription. This provoked tremendous riots and stirred the docile Patriarch Germanos into making a gesture of protest. This didn't exactly deter the emperor; he virtually forced the patriarch in 730 to sign a pro-iconoclast declaration, but Germanus refused and resigned from his post.

To replace him, the emperor named Anastasius, who was previously an official of the patriarchal court. He was a lapdog, pretty much. After Leo died and his son Constantine V took the throne, a rebel named Artabasdus briefly captured Constantinople. Athanasius made the wrong decision: he sided with the usurper. When Constantine came back into the the city in force, he had Athanasius publicly whipped, put backwards on an ass, and ridden around the hippodrome. This humiliation, as you might imagine, was a blow to the prestige of the patriarchate--considering that Constantine let him remain as patriarch. The rest of the period saw ever-growing encroachment upon and humiliation of the patriarchate by the state. This persisted until approximately 780, when the infant Constantine VI took the throne.

Tarasios-Methodius (784-847)

We, therefore ... define with all certitude and accuracy
that just as the figure of the precious and life-giving Cross, so
also the venerable and holy images, as well in painting and mosaic
as of other fit materials, should be set forth in the holy churches of
God ... to wit, the figure of our Lord God and Savior Jesus Christ,
of our spotless Lady, the Mother of God, of the honorable Angels, of
the saints and of all pious people.

-from the Definition of the VII Ecumenical Council

Irene promptly set about restoring orthodoxy. She did not do this altruistically; her political interests were at stake, since she would gain influence and support from the growing icon-restoration movement. She arranged an ecumenical council by framing it as the will of Patriarch Germanus. This council affirmed the veneration of icons, but expressly forbade their worship (the distinction is in the amount of significance ascribed to the icons themselves). Irene named Tarasios, a man from a wealthy patrician family, but who had no clerical connections, patriarch. She wanted a predictable person that she could influence easily, and she got him, since Tarasios had to depend on her support. Irene plotted a lot, mostly against her son. whom she eventually had blinded and killed. However, her clever use of religious issues (her son's controversial remarriage in 795, the return of the relics of St. Euphemia in 796) led Tarasios' prestige and that of the patriarchate as a whole to go up dramatically compared to the previous period--though it still remained tied to the throne.

The next emperor, Nicephorus I, was an usurper who killed and deposed Irene in 802. He was not opposed to orthodoxy; when Tarasios died in 806, Nicephorus appointed another Nicephorus to be the new patriarch. This patriarch was very similar to Tarasios; it is not unlikely that they were related. At any rate, they came from very similar backgrounds. The emperor made every effort to legitimize his patriarchal choice; he was fairly successful. In the first few years of his rule Nicephorus' main concern had nothing to do with icons, but with the restoration of a certain misbehaving bishop. This caused a huge schism, largely along secular/monastic lines, with the Studites and their powerful monastic faction standing squarely against the bishop's pardon. This was generally a poltical issue, though, and was soon taken care of after Nicephorus I died and a short-lived political crisis set in. Michael I, who emerged from it, did not last long, and Leo V , who succeeded him (supported by the iconoclast military), ushered in (like his namesake) a new period of iconoclasm.

Leo V was not, like Leo the Isaurian, interested in religious issues especially deeply. Mainly, he wanted to secure his power, and the military failures of the preceding emperors gave the army a desire to go back to the glorious days of Constantine V, and by extension to iconoclasm. Thus, he was on the army's side, and it was clear that supporting the iconoclasts was another method to gain power at the expense of the patriarchy. He cleverly manipulated the Church from within, using respected theologians as strategic weaponry. When the squabbles that he had seeded finally grew, he stepped in and asserted his power as emperor to establish iconoclasm. The patriarch saw through his plan, but was powerless to stop it, although he and his supporters delivered a number of crushing blows to the notion of the legitimacy of what the emperor was doing. The emperor was forced to resort to repression and violence, discarding any subtlety. Iconoclasm was instituted, but at great cost. to the iconoclasts.

When Leo was killed in 820, he was replaced by Michael II. The latter did not want any trouble; though he stopped the persecution and repression, he did not remove the ruling doctrine of iconoclasm from power. He wanted to save face for the throne. The iconodules' rejoicing was curbed by the deaths of the two greatest theologians in their ranks: St. Theodore Studite in 826 and the former patriarch Nicephorus in 828. Repression began again a year after Michael's son Theophilus took the throne.

In 842, the throne of Byzantium was taken by Michael III (the empress Theodora as regent) and, a year later, the patriarchate was taken by Methodius. He was orthodox; immediately, he began purging the ranks of the Church of any trace of iconoclasm. Twenty thousand clerics were deposed. In 847, with Theodora's help, Methodius triumphantly restored orthodoxy, this time for good. The day (February 19) is still celebrated in the Eastern churches as the Triumph of Orthodoxy.

1. It is completely irrelevant to the discussion whether one believes in Christ or God, or recognizes the tenets of Christianity. The point is, those that were part of this crisis most certainly did, and that is how we must judge these things.
Sources:

Afinogenov, Dmitry. The Constantinople Patriarchate and the Iconoclast Crisis in Byzantium. Moscow: 1997.

Catholic Encyclopedia (http://www.newadvent.org/cathen/)

http://faculty.wm.edu/rbsche/H111_doc_gregoryiitoleoiii.html

http://www.roman-emperors.org

http://church-of-the-east.org/library/BYZDATES.HTM