Around the year 700 AD, the Byzantine Empire was in turmoil and disarray. This was due to continual raids which had drained the empire's resources. This situation changed for the better as Leo III the Isaurian allied with Ardavasdus (strategos of Armeniakon) and seized the throne from Theodosius III in 717. This change in power occurred at a crucial moment, as Constantinople, the capital of the Byzantine Empire, was soon to be invaded by Arabs, Avars, and Bulgarians. The Arabs were the main concern to the Byzantine Empire. Because of the chaos, which had existed in Constantinople since the death of Constantine IV, the Arabs had made sizable advances in Asia Minor and were now ready to strike again. In August of 717, the Arab siege of Constantinople commenced. Leo III cleverly reorganized the empires military forces against the invaders. He led victories in Asia Minor and attacked the Arabs from the rear. While Leo III was leading triumphantly in Asia Minor, Greek Fire was repelling the Arab naval forces.
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Perhaps the main reason for the successful defense of Constantinople was the recent development of Greek Fire. As such, it is necessary to take an in-depth look at the phenomenon, which may have saved the Byzantine Empire from the attack of the Arabs. Greek Fire was a secret weapon of the Byzantine Empire; so secret was this weapon that little is known about the specifics of it to this day. It is rumored to have been developed by Syrian engineer, Callinicus, in 673. The "liquid fire" was shot out of pressurized bronze tubes, which were mounted on the prows of their galleys and on the walls of Constantinople. When Arab ships were hit, they would burst into flames on contact. This could be considered a type of primitive flame-thrower. The secret composition of Greek Fire was handed down through successions of emperors. Although still unknown, it is believed that it was composed of chemicals such as liquid petroleum, naphtha, burning pitch, sulfur, resin, quicklime and bitumen. There was also some other secret ingredient.
After a year of fighting and sustaining heavy losses, the Arabs retreated from Constantinople in August of 718. Leo III had successfully defended Constantinople and his theme system was now completely operational and provided continued strength against future Arab raids, none of which threatened Constantinople again during his reign.