Gregory II began his pontificate; his conflict with Byzantine emperor Leo III eventually led to the establishment of the temporal power of the pope. Pope Gregory II: Pope Gregory II was the bishop of Rome from 19 May 715 to his death on 11 February 731. His defiance of Emperor Leo III the Isaurian as a result of the iconoclastic controversy in the Eastern Empire prepared the way for a long series of revolts, schisms, and civil wars that eventually led to the establishment of the temporal power of the popes. May 19, 715. Wiki link