Marina Borisova writes:
What made Byzantine Christians take up arms against one another in the stronghold of the Orthodox world of the eighth – early ninth centuries? What caused an extremely violent civil war between iconoclasts and icon worshippers, fighting with such ferocity that an Ecumenical Council had to be convened to stop the fratricide. The entire church hierarchy, including patriarchs, suffered from this war.
Iconoclasm, condemned by the seventh Ecumenical Council in 787, arose 60 years earlier under the Byzantine emperor Leo the Isaurian, who banned the veneration of icons. This phenomenon reached its apogee in the reign of Constantine V. Thousands of icons, mosaics, frescoes, statues of saints and altar paintings were destroyed. Icon worshippers were repressed, regardless of their status.
The list of victims includes the Patriarchs Herman I and Nikephoros, as well as the anathematized theologian John of Damascus and many others who were scourged, exiled or executed. Why were Christians so cruelly persecuted in a Christian country just because they honoured their relics? The underlying reasons are not so much ecclesiastical as political.