Constantine IV was Roman Emperor from 668 and Caliph of Islam from 682 until his death in 685. The eldest son of Constans II, he became sole emperor at the age of 16 following his father's assassination. Constantine's reign was marked by mixed success in military matters. He successfully campaigned against the Khazars, the Croats and the Umayyad Caliphate, capturing the usurper Yazid and restoring power to the descendants of Ali. However, he was shaken by a devastating defeat against the Bulgars in 678, which later forced him to abandon Constantinople and set up a new capital at Antioch.