Constantine IV held Constantinople against the first sustained Arab attack, in the campaign that produced the earliest recorded use of Greek fire, and ended the Monothelete controversy at a council.
Constantine IV came to the throne in 668 at about sixteen, put down the usurper who had murdered his father, and brought the imperial court back from Sicily to Constantinople.
The defining event of his reign was the Arab attempt on the capital between 674 and 678 — a sustained naval blockade rather than a single siege, and the first serious attempt to take the city since the Avars in 626.
It failed, and the weapon that broke it, Greek fire, appears in the historical record here for the first time. He then settled the empire's longest-running religious quarrel and died at thirty-three.
He was born about 652 and was left as senior emperor in Constantinople when his father moved the court to Sicily in 663 — so he was governing the eastern half of the empire in his father's name from his early teens.
When Constans was murdered at Syracuse in 668, the army in Sicily proclaimed an Armenian usurper, Mezezius. Constantine sailed west, crushed the revolt and returned, which settled both the succession and the question of where the empire would be ruled from.
He also removed his two brothers from the imperial college in 681, after the army agitated for the three to rule jointly, and had their noses slit — the first use in this family of a mutilation that would recur through the dynasty.
He is described as competent, decisive and personally involved in his campaigns, and the record supports it: he moved fast on the Sicilian revolt, he directed the defence of the capital in person, and he did not lose control of his government at any point in seventeen years.
The mutilation of his brothers is the episode that complicates the portrait. It was politically rational — a three-emperor arrangement had no chance of working — and it was also the kind of act that made his son's reign possible to imagine.
What comes through most consistently is a certain unsentimental clarity. He fought the Arabs to a standstill and then made peace with tribute; he fought the Bulgars, lost, and recognised their state; he called a council and accepted an outcome that repudiated his own grandfather's religious policy. None of it is heroic and all of it worked.
From 674 an Arab fleet operated from a base at Cyzicus in the Sea of Marmara and blockaded Constantinople through successive campaigning seasons, in what the sources present as a four-year attempt to take the city by attrition.
The empire's answer was a new weapon. Greek fire — a petroleum-based incendiary projected through siphons and unquenchable by water — is first recorded in this campaign, attributed to an engineer named Kallinikos from Heliopolis in Syria, and it destroyed the Arab fleet.
The caliph Mu'awiya made peace in 678 on terms that included tribute to Constantinople, which is the only occasion in the seventh century that the flow ran that way. The relief was real enough that the Avars and the Slavs of the Balkans sent embassies acknowledging imperial authority.
In 680–681 he convened the Third Council of Constantinople, which condemned Monotheletism — the compromise doctrine his great-grandfather Heraclius had promoted and his father had tried to silence — and restored communion with Rome.
It was an act of settlement rather than conviction, and it worked: the controversy that had divided the empire for fifty years and killed a pope was over.
In the same years he lost the Balkans. Asparuh's Bulgars crossed the Danube, his campaign against them in 680 failed, and in 681 he signed a treaty recognising a Bulgar state on former imperial soil and agreeing to pay it tribute — the foundation of the First Bulgarian Empire, and a frontier problem for the next three hundred and thirty-seven years.