Constantine IV (; ; 650 – 10 July 685), called the Younger (; ) and sometimes incorrectly the Bearded (; ) out of confusion with his father, was Byzantine emperor from 668 to 685. His reign saw the first serious check to nearly 50 years of uninterrupted Islamic expansion, most notably when he successfully defended Constantinople from the Arabs. His calling of the Sixth Ecumenical Council saw the end of the monothelitism controversy in the Byzantine Empire; for this, he is venerated as a saint in the Eastern Orthodox Church, with his feast day on September 3. Early career The eldest son of Constans II and Fausta, daughter of patrician Valentinus, Constantine IV had been named a co-emperor with his father in 654, almost certainly in Easter (13 April).. His year of birth is unknown, but it's often given as 650. He became emperor in September 668, when news arrived at Constantinople that Constans II had been assassinated in Sicily. The first task before the new Emperor was the suppression of the military revolt in Sicily under Mezezius which had led to his father's death. Within seven months of his accession, Constantine IV had dealt with the insurgency with the support of Pope Vitalian, but this success was overshadowed by troubles in the east. As early as 668 the Caliph Muawiyah I received an invitation from Saborios, the commander of the troops in Armenia, to help overthrow the Emperor at Constantinople. He sent an army under his son Yazid against the Byzantine Empire. Yazid reached Chalcedon and took the important Byzantine center Amorion. While the city was quickly recovered, the Arabs next attacked Carthage and Sicily in 669. In 670 the Arabs captured Cyzicus and set up a base from which to launch further attacks into the heart of the Empire. Their fleet captured Smyrna and other coastal cities in 672. Finally, in 672, the Arabs sent a large fleet to attack Constantinople by sea. While Constantine was distracted by this, the Slavs laid siege to Thessalonica. The Siege of Constantinople (674–678) Commencing in 674, the Arabs launched the long-awaited siege of Constantinople. The great fleet that had been assembled set sail under the command of Abdul-Rahman ibn Abi Bakr before the end of the year; during the winter months some of the ships anchored at Smyrna, the rest off the coast of Cilicia. Additional squadrons reinforced the forces of