Constantine IV (
Greek: Κωνσταντίνος Δ',
Kōnstantinos 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 Heraclius and Tiberius 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.
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