The dynasty that saved Constantinople from the Umayyad siege of 717–718, ruled for eighty-five years, and made iconoclasm imperial policy.
The dynasty is named for a region it did not come from. Leo III was born at Germanikeia in northern Syria, in territory the caliphate had taken, and his family was resettled in Thrace under Justinian II. Isauria is in southern Anatolia; the label appears in later sources and has stuck regardless of the geography.
He rose as a soldier under Justinian II and by 716 commanded the Anatolic theme, the largest and most exposed military province, facing an Umayyad invasion that everyone knew was coming.
He took Constantinople in March 717 from Theodosius III, who abdicated rather than fight him with an enemy army approaching. It was the sixth usurpation in twenty years, and it was accepted because the alternative was worse.
The Umayyad army reached the walls five months after his coronation and stayed for a year. Greek fire destroyed the fleet, an exceptional winter starved the besiegers, and the siege was lifted in August 718.
The dynasty then spent eight decades converting survival into strength. Leo III reorganised the themes, split the oversized commands that had produced so many usurpers, and issued the Ecloga — a short, practical revision of Roman law in Greek rather than Latin.
Constantine V went further and went on the offensive: nine campaigns against the Bulgars, raids across the Taurus into Syria, and the creation of the tagmata, professional regiments stationed near the capital and loyal to the emperor rather than to a provincial general.
From the 720s Leo III moved against religious images, and in 754 Constantine V called a council at Hieria that declared their veneration heretical. Enforcement fell hardest on the monasteries, which were the centres of resistance: monks were humiliated, imprisoned and in some cases killed, and monastic property was confiscated.
The apse of Hagia Irene shows what the policy looked like in practice — a plain cross where a figure of Christ or the Virgin would have stood. It is the best-surviving iconoclast monument in the city.
Almost everything known about the dynasty's theology comes through the writings of the people who defeated it. The Second Council of Nicaea condemned Hieria in 787, iconoclasm was finally abandoned in 843, and the surviving histories were written by the winning side.
Leo IV died in 780 leaving a nine-year-old son, Constantine VI, and a widow, Irene of Athens, who governed as regent. She reversed the religious policy, restoring the veneration of images at the Second Council of Nicaea in 787.
Her son came of age and pushed her aside, and she spent a decade manoeuvring to get back. In 797 she had him blinded — in the same room in which she had given birth to him, according to the chronicles — and ruled in her own name as emperor rather than empress, the first woman to do so.
She was deposed in 802 by her finance minister Nikephoros and died in exile on Lesbos the following year. The dynasty founded on saving the empire ended with a mother destroying her son to keep his throne.
The Isaurians left the state institutionally stronger than they found it: the themes reorganised, the tagmata created, the law rewritten in the language people actually spoke, the capital repopulated after the plague of 747 and its aqueduct rebuilt.
They also left a religious settlement that was condemned as heresy within a generation of the last iconoclast emperor, which is why their reputation had to be reconstructed by modern historians out of what their enemies could not deny.
The most consequential thing they did was the simplest. They stopped the Arab conquest at the walls of Constantinople in 718, and the frontier they established on the Taurus held until the eleventh century.