Flavins Leo III, surnamed the Isaurian, a Byzantine emperor, born in Isauria about 680, died June 18, 741. The son of a farmer who emigrated from Asia Minor to Thrace, he joined the army under Justinian IT., was rapidly promoted, and in 713 was appointed by Anastasius II. to the supreme command of the troops in Asia, where he held the field against the Saracens. When in 716 the crown was seized by Theodosius III., Leo declared him a usurper, outwitted and avoided the Arab general Muslima, marched upon Constantinople, forced him to resign (March, 718), and became himself master of the empire. The Saracens immediately appeared with an immense army and fleet before Constantinople. This siege, under Omar II., the third by the Saracens, lasted two years, and so powerful were the Mohammedan armaments that the provinces expected the downfall of Leo, the western kingdoms heard that a caliph had ascended the Byzantine throne, and two schemes of rebellion were plotted, which were quickly suppressed when the triumph of the emperor was known.

The Arab fleet was routed in two engagements, and partially consumed by the Greek fire, and few of the ships regained the harbors of Syria. In 726 Leo promulgated an edict for the removal of images from all the churches of the empire, and thus inaugurated the party of the iconoclasts, and a conflict of nearly 120 years. He was opposed by Germanus, patriarch of Constantinople, John the Damascene, and John Chrysorrhoas in the East, and by Popes Gregory II. and III. in the West. The iconoclasts were condemned in 732 by a council assembled at Rome; an expedition sent by the emperor into Italy to reduce the cities opposed to the edict failed in its object, and the exarchate of Ravenna was transferred from the Greeks to the Lombards (734). In the East, there was a rebellion in the Peloponnesus and the Cyclades, and a revolt in the capital, the latter of which was quelled only after much bloodshed. The professors in the schools of Constantinople favored the use of images, and the emperor is said to have therefore ordered the library of St. Sophia to be burned. It is more probable, however, that this library of 36,000 volumes was accidentally destroyed in some conflagration.

After the check which his forces experienced in Italy, he transferred Greece and Illyria from the spiritual authority of the popes to that of the patriarchs of Constantinople. The latter years of his reign were occupied with violent wars with the Saracens. An adventurer, who claimed to be Tiberius, a son of Justinian II., was supported by the caliph, and made his entry into Jerusalem in the garb of a Roman emperor. In 739 the Arab general Solyman invaded the Roman territories with 90,000 men, in three divisions, but retreated into Syria after the defeat of one of the bodies in a pitched battle in Phrygia. In 740 an earthquake caused calamities throughout the empire, demolishing a part of the walls of Constantinople, and destroying whole towns in Thrace and Egypt.