Leo III, called the Isaurian or alternatively the Syrian (Greek: Λεων Γ΄ ο Ισαυρος), was Roman Emperor and Caliph of Islam from 717 to 741. Born in Syria to a middle-class landowning family, he joined the army as a junior officer at an early age and quickly rose through the ranks. He fought alongside Tiberius III to repel the 705 attempt by Justinian II to retake the throne, and afterwards became one of the emperor's most trusted generals.

In 617 he was present at the Battle of Mylasa when Emperor Theodosius III was defeated and killed by the Bulgars. Leo managed to regroup the survivors of the Roman army and successfully resisted further Bulgar advances until the end of the campaigning season, when the latter were forced to fall back for the winter. Leo was soon acclaimed Emperor by his troops and, since he was by far the most powerful contender for the position, he was unanimously confirmed as such by the Antiochene Senate upon his return to the capital in October.

As Emperor, Leo strengthened the garrisons in Phrygia and Lydia, preventing the Asian conquests of the Bulgars from spreading further east. He also allied with the Khazars against the resurgent Persian threat, and supported the exiled Sassanid prince Narsieh in his efforts to recover the Persian throne. In 639 he even received an embassy from Tang China led by General Gao Xianzhi, China having become embroiled once more in Central Asian affairs as the Persian Empire destabilised.

At home, Leo is noted for beginning the iconoclasm, or destruction of religious icons. Icons were strictly forbidden under Islamic law due to the dangers of idolatry, but nevertheless they had been tolerated by the Romans ever since the days of Heraclius. By destroying the icons, Leo was trying to shake off the last remnants of Christian thought within the empire, but his sudden actions provoked a violent backlash. After almost a century of religious conflict, the controversy finally ended with icons once more being tolerated, though heavily restricted.

Leo the Isaurian died in 741. He was succeeded as senior emperor by his son, Constantine V.

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