The Second Arab Siege of Constantinople (717–718)

Published on 17 January 2026 at 00:19

The Second Arab Siege of Constantinople (717–718)

Decisive geopolitical confrontation during the early mediaeval period.
Byzantine Emperor Leo III (the Isaurian) successfully defended Constantinople against a massive invasion by the Umayyad Caliphate (Islamic Empire).
Arab forces took advantage of a period of internal anarchy in Byzantium: a rapid succession of weak rulers after the deposition of Justinian II.
The siege represented the supreme effort by the Muslims to crush the Eastern Christian empire and capture its capital.
Arab strategy: combined a massive land army (led by Maslama ibn Abd al-Malik) and a huge naval fleet to blockade and assault the city from both sides.

Key Events and Duration

The siege began in the summer of 717 and lasted over a full year until the summer of 718.
The outcome remained uncertain for much of the time; the city endured intense pressure from land and sea.
Byzantine defenders held the massive Theodosian Walls and used the natural defences of the Golden Horn and Bosporus.

Factors That Turned the Tide for the Byzantines

Greek Fire: A devastating secret weapon – a flammable chemical mixture (likely petroleum-based) that burnt fiercely even on water.
Byzantine ships equipped with syphons projected Greek fire onto the Muslim fleet, setting many vessels ablaze and causing panic.
This destroyed or scattered large parts of the Arab navy and broke the sea blockade.
Severe Winter and Environmental Hardships: The besieging Arab army suffered terribly from an unusually harsh winter (717–718).
Freezing temperatures, heavy snow, storms, and supply shortages led to widespread famine and disease among the invaders.
Many Arab soldiers and sailors died from cold, starvation, and illness.
Strong Leadership of Leo III: Leo III provided determined political and military leadership.
He rallied the citizens, organised defences, maintained morale, and coordinated counterattacks.
Leo also negotiated alliances (e.g., with the Bulgars, who attacked the Arab rear) and used diplomacy to weaken the enemy.
Additional help came from Bulgar attacks on the Arab land forces and from disease outbreaks in the Muslim camp.

Outcome and Immediate Consequences

On 7 August 717, a massive Arab fleet reinforcement was largely destroyed by Greek fire and storms.
By summer 718, the besiegers were exhausted; Maslama ordered a retreat.
The Muslim army and fleet suffered catastrophic losses (estimates of tens of thousands dead).
The siege ended in complete Byzantine victory; Constantinople remained unconquered.

Historical Significance

Halted the eastern expansion of Islam into Europe for centuries.
Prevented the fall of the Byzantine Empire at its most vulnerable moment.
Together with Charles Martel’s victory at the Battle of Tours (732) in the West, it stopped the “great pincers” of Islamic conquest from closing around Christendom.
Stabilised the frontier between the Byzantine Empire and the Islamic world for generations.
Marked the high-water mark of Umayyad expansion; after this failure, the Caliphate shifted focus eastward, and internal issues weakened further large-scale attacks on Byzantium.
Strengthened Leo III’s position, allowing him to launch the Iconoclastic reforms shortly afterward.

Endnotes

Nick R. Needham, 2,000 Years of Christ’s Power Vol. 2: The Middle Ages, p. 16.
Nicaea 325: The Legacy of the Undivided Church, p. 187.
Kenneth Scott Latourette, A History of Christianity, p. 289.
Earle E. Cairns, Christianity Through the Centuries, p. 176.

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