He was Byzantine Emperor and founder of the Isaurian dynasty. In 717, Leo III ascended the throne of Byzantine emperor. He put an end to the Twenty Years' Anarchy, a period of great instability in the Byzantine Empire, and became the founder of the Isaurian dynasty. Under the attack of the Arab Empire with endless expansion, the Byzantine Empire entered an era of nightmare. In 716, 80,000 soldiers of the Arab Empire besieged Constantinople. Like many other emperors of Byzantine Empire, Leo III entered the army and was promoted to the commander of the Anatolic Theme with military merit. After becoming emperor as Leo III, he defeated the attacking army of the Arab Empire with the impenetrability of Constantinople's walls and the use of Greek fire. Leo III successfully defended the Empire against the Arabs and completely drove them out of Anatolia.