Alexander Schmemann, A History of the Orthodox Church

At the end of the seventh century came a new invasion from a Turkic people, the Bulgars, who asserted their authority in provinces settled by Slavs and began a struggle against the empire that lasted many years. As occurred later with the Varangian conquerors of the eastern Slavs, they themselves became Slavicized. Almost at the very gates of the capital, a mighty Bulgar-Slavic state was gradually formed by a nearly uninterrupted war that lasted throughout the whole eighth — or “iconoclastic” — century.