Leo III, Saint, pope (795-816), a Roman; successor of Adrian I. He was attacked about the face and eyes by members of Adrian's family, who hoped to render him unfit for the papacy. Leo recovered and fled (799) to Charlemagne's protection at Paderborn. In 800, Charlemagne went to Rome and conducted a trial during which Leo successfully defended himself against charges of misconduct made by his enemies. On Christmas day, 800, Leo crowned Charlemagne emperor, the event that traditionally marks the beginning of the Holy Roman Empire. Leo's successor, Stephen IV, crowned Charlemagne's son, Louis the Pious, and thus was established the papal claim to the right to consecrate the emperor. In the East-West controversy over the Procession of the Holy Spirit, Leo declared that the Filioque of the creed was dogmatically necessary but liturgically dispensable, and he recommended its omission in the name of East-West unity. Leo did much to beautify Christian Rome. He was canonized in 1673. Feast: June 12.

Leo III (Leo the Isaurian or Leo the Syrian), c.680-741, Byzantine emperor (717-41). He was probably born in N Syria (rather than in Isauria, as once thought). He held diplomatic and military posts before he deposed and succeeded Theodosius III. His accession ended the anarchy into which the empire had fallen since the reign of Justinian II. Leo defended Constantinople against the last Arab siege (717-18), and although he had to contend with Arab attacks in Asia Minor, he succeeded in ending serious Arab threats for nearly two centuries and reorganized the military provinces (themes) of the empire for greater efficiency. His civil code, the Ecloga, written in Greek rather than in Latin, was a practical handbook that had considerable influence in Byzantium. He is also credited with issuing military, maritime, and rural codes. Leo's attack (726) on devotion to holy images began the long struggle over iconoclasm. Riots and rebellions broke out in Greece, while Byzantine rule in Italy (the exarchate of Ravenna and the Pentapolis of Rimini, Ancona, Fano, Pesaro, and Senigallia) began to crumble. The popes Gregory II and Gregory III opposed Leo's iconoclasm and successfully defied his armed expeditions, thus virtually ending Byzantine suzerainty over Rome. Nevertheless, Leo left a revitalized empire to his son, Constantine V. The Isaurian, or Syrian, dynasty, which he founded, ruled the Byzantine Empire until 802.

known as Leo the Isaurian

(born circa 675, Germanicia, Commagene, Syria—died June 18, 742, Constantinople) Byzantine emperor (717–41), founder of the Isaurian dynasty. A high-ranking military commander, he seized the throne with the help of Arab armies who hoped to subjugate the Byzantine Empire. He then successfully defended Constantinople against the Arabs (717–718). Having crowned his son Constantine V coemperor (720), Leo used his son's marriage to cement an alliance with the Khazars. Victory over the Arabs at Akroïnos (740) was crucial in preventing their conquest of Asia Minor. Leo issued an important legal code, the Ecloga (726). His policy of iconoclasm (730), which banned the use of sacred images in churches, engendered a century of conflict within the empire and further strained relations with the pope in Rome.

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