Constantine IV (Greek: Κωνσταντίνος Δ', Konstantinos IV ), (652–685); sometimes incorrectly called Pogonatos, "the Bearded", by confusion with his father; was Byzantine emperor from 668 to 685. He had been named a co-emperor with his father Constans II in 654, and became senior emperor when Constans was assassinated in 668.
The first task before the new emperor was the suppression of the military revolt in Sicily which had led to his father's death. Within 7 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 sent an army under his son Yazid against the Byzantine Empire. Yazid reached as far as Chalcedon and took the important Byzantine center Amorion. Although 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 diverted by this, the Slavs unsuccessfully attacked Thessalonika.
Constantinople survived the Arab siege until 678, when the Byzantines employed Greek fire against the Arab fleet at the Battle of Syllaeum in Pamphylia. This was one of the first times Greek fire was used in combat. The Arabs withdrew, and were almost simultaneously defeated on land in Lycia in Anatolia.
With the temporary passing of the Arab threat, Constantine had to turn his attention to the Church, torn between Monothelitism and Orthodoxy. In November 680, Constantine convened the Sixth Ecumenical Council (also known as the Third Council of Constantinople), reaffirming the Orthodox doctrines of the Council of Chalcedon in 451. This solved the controversy over monothelitism; conveniently for the empire, most monothelites were now under the control of the Umayyad Caliphate. The council closed in September 681.
In 680, the Bulgars under Asparukh, crossed the Danube into nominally imperial territory and began to subject the local communities and Slavic tribes. Constantine IV led a combined land and sea operation against the invaders and besieged their fortified camp in Dobruja. Suffering from bad health, the emperor had to leave the army, which allowed itself to panic and be defeated by the Bulgars. In 681, Constantine was forced to acknowledge the Bulgar state in Moesia and to pay protection money to avoid further inroads into Byzantine Thrace.
His brothers Herakleios and Tiberios had been crowned with him as Augusti at the demand of the populace, but in 681 Constantine had them mutilated so they would be ineligible to rule. At the same time he associated on the throne his own young son Justinian II. Constantine died of dysentery in September 685.
By his wife Anastasia, Constantine IV had at least two sons:
- Justinian II, who succeeded as emperor
- The Oxford Dictionary of Byzantium, Oxford University Press, 1991.