History: Sep.1, 717: The Byzantine Navy defeats the Arabs using Greek Fire

The Second Arab siege of Constantinople in 717–718 was a combined land and sea offensive by the Muslim Arabs of the Umayyad Caliphate against the capital city of the Byzantine Empire, Constantinople. The campaign marked the culmination of 20 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. Theodosios III was Byzantine Emperor from 715 to March 25, 717.

Leo the Isaurian entered Constantinople on March 25, and forced the abdication of Theodosios III, becoming emperor as Leo III. He was immediately forced to attend to the Second Arab siege of Constantinople, which commenced in August.

After wintering in the western coastlands of Asia Minor, the Arab army of up to 150,000 men crossed into Thrace in early summer 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 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 August 15, 718. On its return journey, the Arab fleet was almost completely destroyed by natural disasters and Byzantine attacks.

Leo III the Isaurian, also known as the Syrian (c. 685 – June 18, 741), was Byzantine Emperor from 717 until his death in 741. He put an end to the Twenty Years' Anarchy, a period of great instability in the Byzantine Empire between 695 and 717, marked by the rapid succession of several emperors to the throne.

The siege's failure had wide ranging repercussions. The rescue of Constantinople ensured the continued survival of Byzantium, while the Caliphate's strategic outlook was altered: although regular attacks on Byzantine territories continued, the goal of outright conquest was abandoned. Historians consider the siege to be one of history's most important battles, as its failure postponed the Muslim advance into Southeastern Europe for centuries.