Constantine IV was Roman emperor from 668 to 685. He was the eldest son of Constans II. His reign is memorable for the repulse of the Muslim attack on Constantinople (674-678). The Roman victory was assisted by the timely invention of an incendiary weapon known as Greek fire; but the courage and tenacity of Constantine deserve all credit. This, the first major check to Muslim encroachment, was received with profound relief by the empire and Western Europe. Constantino's statesmanship was shown in his convocation of the Sixth Ecumenical Council at Constantinople (680-681), in which the orthodox doctrine of the two wills and the two energies of the Savior was upheld and the Monothelite heresy condemned. The one disaster of the reign was a Roman defeat at the hands of the Bulgars (680), which resulted in the establishment of a Bulgar state on Roman soil. Constantine died in Constantinople on July 10, 685, at the age of 33.