Theodosius III in the context of "Byzantine Senate"

In 717–718, Constantinople, the capital of the Byzantine Empire, was besieged by the Muslim Arabs of the Umayyad Caliphate. The campaign marked the culmination of twenty years of attacks and progressive Arab occupation of the Byzantine borderlands, while Byzantine strength was sapped by prolonged internal turmoil. In 716, after years of preparations, the Arabs, led by Maslama ibn Abd al-Malik, invaded Byzantine Asia Minor. The Arabs initially hoped to exploit Byzantine civil strife and made common cause with the general Leo III the Isaurian, who had risen up against Emperor Theodosius III. Leo, however, deceived them and secured the Byzantine throne for himself.

After wintering in the western coastlands of Asia Minor, the Arab army crossed into Thrace in the early summer of 717 and built siege lines to blockade the city, which was protected by the massive Theodosian Walls. The Arab fleet, which accompanied the land army and was meant to complete the city's blockade by sea, was partly neutralized soon after its arrival by the Byzantine navy through the use of Greek fire. This allowed Constantinople to be resupplied by sea, while the Arab army was crippled by famine and disease during the unusually hard winter that followed. In spring 718, two Arab fleets sent as reinforcements were destroyed by the Byzantines after their Christian crews defected, and an additional army sent overland through Asia Minor was ambushed and defeated. Coupled with attacks by the Bulgars on their rear, the Arabs were forced to lift the siege on 15 August 718. On its return journey, the Arab fleet was almost completely destroyed by natural disasters.

Anastasius II (Greek: Ἀναστάσιος, romanized: Anastásios; died 719), born Artemius (Greek: Ἀρτέμιος, romanized: Artémios), was the Byzantine emperor from 713 to 715. His reign was marked by significant religious and political decisions aimed at stabilizing the Empire. One of his notable actions was reversing the previous appointment of a Monothelete patriarch of Constantinople, instead reinstating Orthodoxy by appointing Germanus I to the position. This move was made in an effort to gain the favor of Pope Constantine.

Anastasius II dedicated his efforts to fortifying the Empire and fostering a period of stability. However, his reign was cut short when he was deposed by Theodosius III during the Byzantine campaign against the Umayyad Caliphate in 715. Undeterred, four years later, in 719, Anastasius launched a rebellion against Leo III the Isaurian in a bid to reclaim the imperial throne.