Leo III (ca. 680-741), called the Isaurian, was Byzantine emperor from 717 to 741. He rescued the empire from disaster and began the containment of the Arabs' eastern advance. He also initiated the controversial Iconoclastic movement.
Leo was born in Germanicea in northern Syria, but his family was resettled in Thrace by one of the population movements of the last Heraclian emperor, Justinian II, during his initial regime (685-695). Leo attracted this sovereign's attention during his second regime (705-711) and then won a reputation for daring exploits during the chaos of the Heraclian dynasty's collapse. Under the insecure Anastasius II (713-715) he was advanced to command of the Anatolikon Theme, one of the military provinces of Asia Minor. Rivalries between troop units of the various themes produced rebellions and instability, encouraging Leo's own ambitions. Leo formed a coalition with another thematic commander, Artavasdus, who became his son-in-law. Evading Arab attacks, Leo captured the weak puppet emperor Theodosius III (715-717). He then entered Constantinople, where he was crowned emperor on March 25, 717.
Though a usurper, Leo III proved the right man for the times. Having failed in earlier efforts to overrun the "Empire of the Christians" and to take its great capital, the Arabs under their Umayyad rulers had again taken the offensive. From 695 on, exploiting the chaos attending the Heraclian collapse, the Arab armies plunged deep into Asia Minor and threatened Constantinople directly. With only the briefest time for preparations, Leo faced a full-scale siege as the Arabs began, on Aug. 15, 717, to attack Constantinople with all the land and sea forces the caliphate could muster. Leo's leadership was brilliant. As with the previous Arab siege, under Constantine IV 40 years before, the combination of strong fortifications, excellent organization, and the fearsome secret weapon known as "Greek fire" gave the Byzantines success. The Arabs withdrew in disarray in August 718. This great triumph against the final tide of the Arab offensive saved not only the Byzantine Empire but all of eastern Europe, and perhaps more beyond, from Moslem conquest.
The Arabs' menace to the empire did not end altogether, for during the 720s and 730s they resumed their offensive into Asia Minor. Leo devoted further efforts against them, and in 740 he won a great victory at Akroinon which further crippled the Arabs' position, enabling his son and successor to take the offensive against them. Not only was the capital freed from danger, but the safety of Asia Minor, the empire's greatest source of manpower and revenue, was secured.
Leo III advanced the system of the themes (administrative divisions of the empire) to a significant extent. He reconsolidated the system by dividing the original themes into smaller units and reorganizing them; he had learned from his own success how easy it was for a commander of a large territory to seize the throne. A number of new institutional codifications marked his reign. By far the most important of these is his Eklogá. Issued in 726, this is a digest of essentials from Justinian I's old Corpus juris civilis, but now in Greek, the empire's functional language. This code demonstrated the continuing evolution of Roman law in the East, amalgamated with new Christian and Oriental elements. Leo's work is therefore a bridge between the legal landmarks of Justinian's age and the mature Byzantine codifications of the late 9th century.
Leo's most controversial legacy was the initiation of a campaign against the use of images (icons) in the Church. Though the movement may have been aimed indirectly at the centers of icon support, the monasteries, as parasitical drains on the empire's wealth and manpower, a genuine religious and theological concern cannot be overlooked. Influences of Islam, Judaism, and even some Christian heresies have been suggested as affecting Leo's movement, but it may also reflect a puritanical Asiatic reaction to the Greek philosophical rationalization of physical representation of the Divine.
Leo moved cautiously to discredit images in 726, and he promulgated a formal decree against them only in 730. Popular reaction was strong at home, and further opposition came from the papacy in Rome. Leo's power over distant Italy was limited, and he retaliated by transferring important provinces from Roman ecclesiastical jurisdiction to that of the see of Constantinople. (This move has also been assigned by some historians to Leo's son, Constantine V.) Such friction between pope and emperor only encouraged the papacy's drift into its epoch-making alliance with the Carolingian Franks. Nevertheless, Leo left the full-scale pursuit of Iconoclastic policy within the empire to his successor.
Originally promising the succession to Artavasdus, Leo secured a dynastic line of his own when his son, Constantine, was born in 718. Leo made him coemperor in 720, guaranteeing this family succession. In 733 Leo married Constantine to a princess of the Khazars, thereby winning these Turks as valuable allies against the Arabs in the Caucasus. Leo shared command and authority with Constantine in his last years, so that power passed directly to his son when he died on June 18, 741.
Useful general accounts of Leo's reign are in J. B. Bury, A History of the Later Roman Empire: Arcadius to Irene (2 vols., 1889); George Ostrogorsky, History of the Byzantine State (1957; 2d ed. 1969); Romilly Jenkins, Byzantium: The Imperial Centuries (1966); and The Cambridge Medieval History, vol. 4, 2 pts. (2d ed., 1966-1967). The fullest study of the religious issues is Edward James Martin, A History of the Iconoclastic Controversy (1930).