 Complete the following phrase:  The Romans did not regard the disorganized Arab tribes of the Arabian Desert as potentially dangerous enemies early in the 7th century. However, the Arabic Ghassanids and Lakhmids had been deeply involved in the RomanPersian wars, and the Arabization of the Near East had started. Arab tribes settled in thinly populated lands near the RomanPersian frontier, and the thriving Syrian and Mesopotamian cities attracted Arab settlers. The situation changed rapidly with the unification of the desert tribes by the Islamic prophet Muhammad between 622 and 632. After he died, his monotheistic movement was led by his successors, styled caliphs, who were both the political and religious leaders of the Islamic community. Conducting the jihad, or Islamic holy war, against the infidels, the Arabs attacked the Syrian Roman territories and conquered Damascus in 633. Heraclius deployed a Roman field army against the Arabs, but they annihilated the Roman army on the Yarmuk River in Syria in 636. As the war against Persia had exhausted the empire, Heraclius could not send a second field army against the invaders and they conqered Syria. Two years later, the Arabs inflicted a major defeat on the Persians 