From the 630's, the Arabs penetrated into the territory of the Byzantine Empire and occupied Syria, Palestine, and Egypt by the 640's. The Arabs also invaded the Sasanian Empire and completely annexed Sasanians in 651/52, and as a result they formed a vast and powerful empire. But the emergence and the expansion of the Arabs had another significance : the Arabs also created new Islamic civilization. In the first phase of the Arab invasion during the 630's, the Byzantines could not fully understand the Arabs. The Arabs had been called "Sarakenoi (Saracens)" and identified with "Agarenoi (descendants of Hagar)" or "Ishmaelitai (descendants of Ishmael)" from Late Antiquity, but the word Sarakenoi (and also Agarenoi and Ishmaelitai) did not connote any religious meaning. So during the 630's, the Byzantines (and other Christians and Jews) could not understand the Arabs' faith adequately, and often regarded the Arabs as simply pagans. The Byzantines also expected that the Arabs would disappear in a short time. Because the Byzantines bore yet in mind that the Sasanians had invaded and ruled Syria and Egypt during the 610 - 620's but their rule ended abruptly due to the striking victory of Byzantine Emperor Heraclius (r. 610-641) in 628. However, as the Arab state became stable, and knowledge of the Arabs increased, the Byzantines and Christians outside of the Byzantine Empire gradually understood that the Arab state was much more powerful than the Byzantine Empire, and that the Arabs had been developing their own faith. The first Arab attack against Constantinople in 654 is of particular importance. In 654, after the preparations of a much larger number of troops and an armada, the Arabs penetrated into the Byzantine Empire and quickly reached Constantinople. But the Arab fleets suffered considerable damage as the result of a heavy storm, and their land forces were also forced to withdraw. This incident is important in two respects. At first, the Byzantines became more conscious of their own weakness and Arab strength. Therefore from the 650's, the Byzantines immediately began the reforms of their defenses and administration to respond to this serious situation. Secondly, the Arabs had been defeated and withdrew because of the heavy storm. This was interpreted to mean that Byzantium and the Byzantine Emperor had received God's special providence. From the 630's, Emperor Heraclius and his grandson Constans II (r. 641-668/69) had been heavily criticized due to their monenergist and monothelete doctrines, and their critics like Maximus the Confessor asserted that the Arabs' rapid expansion and the defeat of the Byzantines had been caused by the emperor's heretical policy : Maximus and his supporters insisted that the sinful religious policy of Heraclius and Constans II had provoked God's anger. However, the "victory" in 654 proved that not only Constantinople and the Byzantine Empire but also the Emperor had been receiving God's protection. In other words, the "victory" over the Arabs in 654 proved the Emperor's religious doctrine was right. Confrontation with the Arabs continued from 660's onwards, and the Arabs again intensified their efforts to conquer the Byzantine Empire. But the Byzantines withstood tenaciously and even began to counterattack the Arabs in the 670's. Under such circumstances, the Byzantines further reinforced the belief that the Byzantine Empire and the Emperor received God's Providence. For them, the Roman Empire meant the Christian community ruled by an Emperor for whom God provided divine protection. On the other hand, the Arabs could not extinguish the Byzantine Empire, and they were forced to coexist with Byzantium over a long period. So, the Arabs could not become the sole 'heirs' of the Roman Empire and Roman civilization, and on account of this, they chose to form their own Islamic civilization from the end of the seventh century.