The wealth and strength of this inheritance was tested in the 7th century when the Arabs conquered most of the eastern Mediterranean and the Persian empire. The most prosperous Roman provinces of Syria, Palestine and Egypt were overrun. By 711 Arab forces had advanced to the Pillars of Hercules and sailed over into southern Spain, which they occupied until 1492. In the east they crossed the Oxus and moved into Uzbekistan, which of course remains Muslim to this day. In 717 the Arab navy sailed into the Sea of Marmara, while two land armies from Damascus camped outside the walls of the capital and prepared to starve it into surrender. The newly enthroned emperor, Leo III, was an experienced military leader who called on Khazar allies from the Crimea to attack the besiegers in the rear while he made strategic use of Greek fire to destroy enemy ships and led military sorties from within the city. Fortunately for Byzantium, the winter of 717-18 was extremely harsh and by the following summer the caliph had ordered the siege to be lifted. During their retreat the Arabs suffered storms in the Aegean which destroyed their ships. This great Byzantine triumph was celebrated annually on August 15, already a most holy day as the feast of the Dormition (Assumption) of the Virgin.

The Arab challenge, however, was not exhausted. Constantinople was the great prize that all Muslim caliphs wanted to win, to make it their capital, from which they could advance into Europe to replace Rome and make good their claim to be the true successors to the followers of Christ. Between 719 and 740 their attacks continued unabated. The defence constitutes one of Byzantium's historic claims: it held off the Arab threat to conquer and occupy Constantinople and thus protected the west from Muslim invasion until the 14th century. While Charles Martel, "the hammer", defeated the Arabs from Spain in 732 at Poitiers and pushed them back over the Pyrenees, Byzantine forces protected Greece, the Balkans, Italy and central Europe for centuries during times of fragmented political rule in the west. This stalwart and prolonged military achievement permitted the powers of western Europe to develop and consolidate, notably through the short-lived empires of Charlemagne, the Ottonians and the Hohenstaufens, Frederick I and II. Without Byzantium blocking the Arab advance, this could never have happened.

But an indirect consequence of the earlier and most challenging Arab campaigns in the east was a tremendous upheaval within Byzantium itself, directed towards the images of holy persons, icons of Christ, the Mother of God, saints, monks and bishops. As Muslim victories over the Christian forces of Byzantium dominated the first half of the 8th century, churchmen and civilians alike questioned why Byzantine forces were not more successful, and concluded that they must be committing some very grave sin. The emperor's advisers informed Leo III that it was the sin of idolatry - excessive worship of Christian icons - that was causing divine displeasure. So in 730 he issued an order to remove the holy images, thus opening a period of iconoclasm (the destruction of icons), which dominated the empire until 787, and again from 815-43. The key to Byzantine iconoclasm, however, must lie with the example of Islam, which admitted no figural representation in full compliance with the Second Commandment of Moses: "Thou shalt not make unto thee any graven image, or any likeness of any thing" (Exodus 20:4-5).