How a desperate Byzantine gamble saved Europe from a century of Arab expansion
In 717, the Umayyad Caliphate launched a massive, well-planned assault to extinguish the Byzantine Empire. By cutting off Constantinople from land and sea, they aimed to decapitate the state. Instead, a harsh winter, internal deception, and the terrifying power of Greek fire turned the siege into a catastrophic defeat.
The 717–718 siege was the culmination of two decades of Arab encroachment into Byzantine borderlands. Exploiting internal Byzantine turmoil, the Umayyad Caliphate sought to capture the capital, believing its fall would make the remaining provinces easy to seize. The Arab forces, led by Maslama ibn Abd al-Malik, were vast, with some medieval accounts claiming up to 200,000 men and 5,000 ships. They arrived prepared for a long conflict, bringing siege engines, incendiary materials, and even wheat to sow in the fields surrounding the city.
The defense was led by Emperor Leo III the Isaurian, who had secured the throne through a series of ruses and political maneuvers. While the Arab army built double siege walls to isolate the city, the Byzantine navy utilized Greek fire to neutralize the Arab fleet, keeping the capital's supply lines open. As the winter of 718 proved exceptionally harsh, the Arab camp suffered from famine and disease, with soldiers reportedly resorting to eating livestock and even cannibalism. When reinforcements arrived in the spring, their Christian crews defected, leading to the destruction of the new fleets and the defeat of an overland army.
The failure of the siege had profound geopolitical consequences, marking the end of a century of constant war and territorial losses for Byzantium. Historians consider it a pivotal moment that postponed Muslim expansion into Southeastern Europe for centuries. For the Umayyad Caliphate, the disaster was severe; the loss of their fleet and elite Syrian troops undermined the material basis of their power. While raids continued, the strategic goal of outright conquest was abandoned, and the Caliphate eventually faced internal instability that culminated in the Abbasid Revolution.
The siege remains a subject of historical study because primary sources are often contradictory and colored by later biases. Byzantine accounts, such as those by Theophanes the Confessor, often favor Leo III, while Arab records like those of al-Tabari contain legendary elements. Despite these discrepancies, the logistical failure of the Arabs—operating far from their Syrian bases—combined with the strength of the Theodosian Walls and Byzantine naval technology, remains the consensus explanation for the empire's survival.
