Though capable of fratricidal cruelty, Christianity owes Constantine IV, an enormous debt. It was Constantine who, by a mixture of military genius and luck, finally halted the Islamic advance westward. After Constantine's victories, all the Islamic incursions into Europe until the eventual fall of Constantinople were forced to stretch their supply routes thin and strike far west.

Born about 650, Constans II raised his eldest son Constantine to the rank of co-emperor in April 654. Though a child then, by the time of his father's murder he was a capable teenager. In early 669 he led an army to Sicily and defeated his father's killer, Mezizios. For the next decade, Constantine ruled with his two younger brothers, Heraclius and Tiberius as junior emperors. This arrangement was not to last though. In 681, Constantine deposed and mutilated his brothers and took sole authority on himself.

Constantine's greatest challenge came from the Arabs, who had been raiding imperial territory yearly, and in the five years, 674 to 678, laid an intense naval siege of Constantinople itself. A lesser man might have folded under such intense pressure, but Constantine proved himself a genuine leader. With the aid of a new secret weapon, the legendary "Greek fire" - a flammable mixture of oil and tar capable of catapulted or siphoned over enemy ships like a medieval napalm - the Arab commander Mu'awiya, was compelled to sign a thirty-year truce with the empire and cede the islands he had taken.

Though adept at war, Constantine was inclined towards diplomacy. After his defeat of the Arabs, he signed treaties with the Lombards in Italy and the Avars to the north. He launched an unsuccessful campaign against the Bulgars - the army panicked on the rumour the emperor had abandoned them when he had only been stricken with gout - and recognized the Bulgars' settlement south of the Danube, something that would have happened sooner or later anyhow.

Constantine made great headway in heading the empire's religious disputes by summoning the Council of Trullo. He personally presided over twelve of the eighteen meetings and endorsed the near unanimous verdict of the Council, effectively ending the monothelite divide. In the coinage, he revalued the follis, producing a large 18 gram issue. His gold coinage revived military iconographic themes not in over a century, (compare the obverse of the solidus above with Justinian I's follis.)