Siege of Constantinople (717–718)

For about a year, from the summer of 717 to 15 August 718, a great army and fleet of the Umayyad Caliphate besieged Constantinople, capital of the Byzantine Empire. It was the second Arab attempt on the city and the most dangerous, aimed at taking the capital and with it the rest of the empire. Its failure, brought about by Greek fire, a brutal winter, defecting sailors and Bulgar attacks, ended the caliphate's hopes of conquering Byzantium outright.

After a lull following the first Arab siege of 674–678, war resumed in 692. Arab armies took Armenia and the Caucasus and raided ever deeper into Asia Minor, while Byzantium was weakened by a long run of coups: between 695 and 717 the throne changed hands seven times. The caliph al-Walid I began planning a new assault on Constantinople, and his brother and successor Sulayman pressed it forward. Arab accounts say he was drawn by a prophecy that a caliph bearing a prophet's name would take the city. Command went to another brother, Maslama ibn Abd al-Malik.

The Arab advance began in 715, just as Byzantine troops overthrew Emperor Anastasius II in favour of Theodosius III. Maslama dealt with Leo, the general commanding the Anatolic theme, hoping to deepen Byzantine divisions. Leo instead played the Arabs off against his rivals, garrisoned the fortress of Amorium for himself, and marched on the capital. Theodosius abdicated, and Leo entered Constantinople as Leo III on 25 March 717, with time to prepare its defences.

In early summer 717, Maslama crossed into Thrace and reached the city in mid-July or mid-August, enclosing it on the landward side with a double wall facing both the city and the countryside. Ancient figures for his forces, such as 120,000 men and 1,800 ships, are certainly exaggerated, but the attackers far outnumbered the defenders, perhaps no more than 15,000 men. The Arab fleet arrived on 1 September. As it entered the Bosporus, a change of wind left part of it near the walls, where Byzantine ships destroyed it with Greek fire, an incendiary weapon. Leo then closed the Golden Horn with a chain. The surviving fleet kept its distance, so the city could still be supplied by sea.

The winter of 717–718 was exceptionally harsh, with snow lying for more than three months. Food ran out in the Arab camp, and soldiers were reduced to eating their animals, roots and bark; disease followed. In spring the new caliph, Umar II, sent two supply fleets from Egypt and North Africa, but many of their Christian Egyptian crews deserted to the Byzantines, and Leo's ships destroyed or captured the rest. A relief army marching through Asia Minor was ambushed and routed near Nicomedia. Meanwhile the Bulgars, with whose ruler Tervel Leo had made a treaty, attacked the besiegers; one Byzantine chronicler claims they killed 22,000.

Umar ordered the siege abandoned, and the Arabs withdrew on 15 August 718, the feast of the Dormition of the Virgin, to whom the Byzantines credited their deliverance. Storms and volcanic ash from Thera destroyed much of the retreating fleet. Arab sources speak of 150,000 dead, a figure modern historians regard as inflated but telling of the scale of the disaster.

The defeat strained the Umayyad state and its finances and cost it much of its Syrian army and fleet. Raids on Byzantium resumed in 720, but now for plunder rather than conquest, and the frontier settled along the Taurus Mountains for more than two centuries. Together with the Frankish victory at Tours in 732, the siege is often seen as having checked Muslim expansion into Europe until the 15th century, when the Ottomans achieved the fall of Constantinople. In Arab legend, by contrast, the defeat was later reimagined as a victory.

Our 700 map shows the Eastern Roman Empire, the Umayyad Caliphate and the Bulgars across the Danube on the eve of the siege, with Constantinople in today's Turkey.