15 August

The Second Arab siege of Constantinople (717–718) was finally raised after a year of conflict.

The Arab fleet, which accompanied the land army and was meant to complete the city’s blockade by sea, was neutralized (with Greek Fire) and 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 and Byzantine attacks.

The siege’s failure had wide-ranging repercussions. The rescue of Constantinople ensured the continued survival of the Eastern Roman Empire (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.