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The followers of Muhammad had invaded the Byzantine Empire in the mid-seventh century. The Muslims defeated the army of the Christian Emperor at the Battle of Homs. After this victory, the Arabs conquered the Byzantine territories in the Near East and North Africa. A Muslim navy and army besieged Constantinople between 674-678 in alliance with the Avars but were ultimately defeated.<ref> Treadgold, Warren T. <i>[https://www.amazon.com/gp/product/0804726302/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0804726302&linkCode=as2&tag=dailyh0c-20&linkId=82b3332950b0a9c88f488ff1ac4c672a A History of the Byzantine State and Society ]</i> (Stanford, Stanford University Press, 1997), p 16</ref> There then followed several decades of an uneasy peace, between the two superpowers. However, by 700 AD the Umayyad Caliphs were at the peak of their power and ruled an Empire from Spain to Central Asia. They began large-scale incursions into the Byzantine heartland and also conquered the Christian allies of the Empire in the Caucuses.

<a target="_blank" href="">A History of the Byzantine State and Society</a><img src="//ir-na.amazon-adsystem.com/e/ir?t=dailyh0c-20&l=am2&o=1&a=0804726302" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /> By the early years of the 8th century, the Umayyad Caliph was preparing for the final destruction of the Christian Empire. Constantinople was the key, and the Arabs knew that they had to capture the city if they were to conquer the successor of Rome.<ref>Treadgold, p. 87</ref> Byzantine had been severely weekend by a series of civil wars and its eastern defenses were crumbling. The Arabs launched a combined land and sea invasion in 716 AD. They followed a coastal route and by-passed many Christian strongholds. To ease the conquest, they encouraged one of the governors in Anatolia to revolt and claim the throne. The reigning Byzantine Emperor abdicated rather than plunge the Empire into another civil war. The rebel governor became Emperor Leo III and is popularly known as Leo the Isaurian. He had agreed with the Arabs that if he became Emperor that he would become the vassal of the Umayyad Caliph. Leo III did not keep his end of the bargain and was determined to keep Byzantine independent and Christian. Enraged the Caliph Umar II ordered his massive army and navy to the vast walls of Constantinople.