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In Autumn 717 the Caliph Umar II ordered his brother Masalama to besiege Constantinople. He led an army of 80,000 and a fleet of some two hundred ships, mostly oared galleys.<ref> Davis, Paul K. "Constantinople: August 717–15 August 718"<i>[https://www.amazon. com/gp/product/0195143663/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0195143663&linkCode=as2&tag=dailyh0c-20&linkId=af9472455d3cb5895f9d3b9f349c6f4a 100 Decisive Battles: From Ancient Times to the Present ]</i> (Oxford, United Kingdom: Oxford University Press, 2001). pp. 99–102</ref> The Caliph personally directed the attack from Damascus. Leo III was an experienced soldier and he had strengthened the walls of the city and had stockpiled supplies. He ordered any civilian who did not have three years supply of food to leave the city. The Byzantine navy was able to keep the Bosporus open despite continued efforts of the Arab fleet to blockade the waterway and cut off the city from the Black Sea. The Christian navy had a secret weapon, and this was ‘Greek Fire.’ <ref> Davis, p. 100</ref> This was a flammable liquid that could be directed upon the enemy with devastating effects. The Byzantines used Greek Fire in a similar manner to a modern flamethrower.

This weapon allowed the Christian ships to destroy many Arab galleys. The Byzantines received regular supplies by sea, but the Arabs suffered great hardship, especially in winter. The Caliph sent reinforcements from Egypt. Despite many assaults, the Arabs could not breach the walls of Constantinople. Leo III then entered an alliance with the Bulgars, a Turkish people who had established a powerful kingdom on the borders of Byzantium.<ref> Davis, p. 101</ref> They agreed to attack the Arab forces besieging the capital of the Christian Empire, in return for payments of gold. In the summer of 718 AD, the Bulgars attacked the Arabs and destroyed a 5000-strong foraging party. The Umayyad forces were almost trapped between the walls of Constantinople and the Bulgars. The Caliph ordered a retreat from Constantinople but during this, the Bulgar Khan's forces massacred some 30,000 Muslim soldiers before they could board the ships. The Byzantine navy pursued the Muslim navy and destroyed many galleys, many more Muslim ships were lost in a storm.<ref>Davis, p. 101</ref>