Siege of Constantinople (717–718)

Lore & Background

In 716, after years of preparations, the Arabs under 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 early summer 717 and built siege lines to blockade the city, which was protected by the massive Theodosian Walls.

Reader's Guide

The siege's failure had wide-ranging repercussions. The rescue of Constantinople ensured the continued survival of Byzantium and marked the end of a century of constant war, territorial losses and internal strife, 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. The Byzantine navy's use of Greek fire neutralized the Arab fleet, allowing the city to be resupplied by sea, while the Arab army was crippled by famine and disease during the unusually hard winter.

Did You Know?

• The Arab fleet was partly neutralized soon after its arrival by the Byzantine navy through the use of Greek fire.
• In spring 718, two Arab fleets sent as reinforcements were destroyed by the Byzantines after their Christian crews defected.
• 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.

The Political Gambit of Leo III

In 716, Umayyad forces under Maslama ibn Abd al-Malik swept into Byzantine Asia Minor, seeking to exploit a empire in disarray. The Byzantine throne had changed hands seven times through violent coups following Justinian II's first deposition in 695, and the defensive frontier was visibly crumbling under repeated Arab raids. Into this chaos stepped Leo III the Isaurian, a general who had risen in revolt against Emperor Theodosius III. The Arabs, recognizing what they believed to be a useful ally, made common cause with Leo, expecting to channel the civil strife toward their own strategic ends. Leo, however, executed a calculated deception: he feigned alignment with the Umayyad cause while quietly maneuvering to place himself on the Byzantine throne. Once he had secured imperial power, the Arabs found their political lever in Constantinople rendered useless. The betrayal did not, however, halt the campaign. Already committed and assembled, the Arab army and fleet pressed forward toward the capital, setting the stage for the great siege of 717–718.

Greek Fire and the Breaking of the Naval Blockade

The Arab strategy for taking Constantinople rested on total encirclement. The land army would press against the massive Theodosian Walls while a large fleet sealed the city's maritime approaches, cutting off all resupply. After crossing into Thrace in early summer 717, the besiegers established their siege lines and positioned their ships to complete the blockade. The Byzantine navy, however, struck quickly and effectively. Through the deployment of Greek fire, the defenders partially neutralized the Arab fleet almost immediately after its arrival, preserving the sea lanes that kept Constantinople provisioned. This single tactical success denied the besiegers the starvation strategy they had counted upon. The naval threat was then systematically dismantled in spring 718: two Arab reinforcement fleets were destroyed after their Christian crews defected to the Byzantine side, and a further overland army sent through Asia Minor was ambushed and defeated. With the sea dimension of the siege broken, the Arab land force stood isolated and increasingly vulnerable.

The Winter of Misery and the Final Collapse

The unusually severe winter of 717–718 proved catastrophic for the Arab besiegers. Deprived of naval support and unable to secure adequate provisions, the army was ravaged by famine and disease to the point of crippling its operational capacity. The situation deteriorated further when the Bulgars launched attacks on the Arab rear, threatening lines of communication and any orderly retreat. By the following spring, the combined pressures of starvation, illness, and external military threats rendered continued operations untenable. The Arab command lifted the siege on 15 August 718, a withdrawal that was itself nearly fatal. On the return journey, the Arab fleet was almost entirely destroyed by natural disasters, compounding military defeat with a near-total loss of naval assets. What had begun as a vast, carefully prepared invasion—personally supervised by Caliph Sulayman at the assembly ground of Dabiq north of Aleppo—ended in a rout that saw reinforcement fleets annihilated, overland armies defeated, and the fleet itself scattered by storms.

A Century of War Brought to an End

The failure of the 717–718 siege reshaped the strategic landscape of the eastern Mediterranean for generations. For Byzantium, the defense of Constantinople guaranteed the empire's continued existence and brought to a close a century marked by relentless warfare, progressive loss of border territory in Armenia and the Caucasian principalities, and repeated internal upheaval. The Arab advance that had steadily eroded Byzantine frontiers for twenty years was halted at the very gates of the capital. For the Umayyad Caliphate, the defeat fundamentally recalibrated its strategic outlook: while periodic raids into Byzantine lands continued in the decades that followed, the overarching ambition of outright conquest was abandoned. Historians regard this siege as one of the most consequential battles in recorded history precisely because its failure postponed the Muslim advance into Southeastern Europe for centuries. The campaign had been the culmination of a long, grinding encroachment, and its collapse reversed the momentum that had threatened to extinguish the Byzantine state entirely.