From August 717 to August 718 an Umayyad army and fleet blockaded Constantinople by land and sea. Greek fire, an exceptional winter, mass defections and a Bulgar attack destroyed the expedition, and the caliphate never tried again.
The Umayyad caliphate attacked Constantinople in August 717 with an army under Maslama ibn Abd al-Malik, brother of the caliph Sulayman, and a fleet large enough to close the sea approaches. It was not a raid. The intention was to take the city and end the Roman empire, and the preparation had run for years.
The city was held by Leo III, who had been emperor for five months. He had taken the throne in March from Theodosius III, and the reason the empire accepted a usurper that spring was that he was the general most likely to survive the summer.
The siege lasted twelve months and destroyed the attacking force. Greek fire broke the fleet, an exceptionally hard winter starved the army, the reinforcing squadrons of 718 lost their crews to desertion, and a Bulgar army fell on the besiegers from behind. The caliphate never attempted the city again.
By 717 the Arab conquests had taken Syria, Egypt, North Africa and Spain, and the Roman empire had been reduced to Anatolia, the Aegean and fragments of Italy. Constantinople had already been raided and blockaded in the 670s, and the Umayyads had learned that a fleet alone could not do it.
Sulayman prepared accordingly: a land army marching through Anatolia, and a fleet to cut the city from the sea and prevent grain reaching it. The two arms together were the correct plan, and it was the plan that the Avars and Persians had failed to execute in 626 for want of ships.
On the Roman side the empire had spent twenty years in political collapse, with six emperors between 695 and 717. Leo, strategos of the Anatolic theme, negotiated with Maslama during the Umayyad advance and then used the time to take Constantinople for himself. Theophanes presents this as Leo outwitting Maslama with false promises; the Arabic tradition presents Maslama as having been deceived by an ally who then turned. Both accounts are written to explain a failure or a success, and neither can be checked.
Maslama crossed the Dardanelles at Abydos in July 717 and reached the land walls in August, where he dug a ditch and built a wall of his own facing the city — investing the defenders and covering his own rear against attack from Thrace.
The fleet arrived in September and moved to close the Bosphorus and the Sea of Marmara, which was the decisive part of the plan. If the water was shut, the city would starve; the Theodosian walls had never been forced, but they could not feed anyone.
The garrison's response was the same as in 626, and it worked for the same reason. The chain was drawn across the Golden Horn, the imperial fleet stayed inside it, and it came out to fight when the besieging ships were divided by the currents.
As the Umayyad squadrons moved past the city to take station in the Bosphorus, the Byzantine fleet attacked them with Greek fire — a petroleum-based incendiary pumped through bronze siphons mounted in the bows, which burned on water and could not be put out with it.
The composition was a state secret and remains unrecovered; what the sources describe consistently is the effect, and the effect in September 717 was that a substantial part of the Umayyad fleet was destroyed or scattered in a single action. The survivors withdrew out of range.
That single engagement decided the campaign, though it took another eleven months to become obvious. The blockade by sea was never fully re-established, supplies continued to reach the city, and an army built to besiege was left sitting outside a city better fed than it was.
The winter of 717–718 was exceptionally severe, and the sources say snow lay on the ground for a hundred days. The besieging army had provisioned for a short campaign against a city expected to fall.
What follows in Theophanes is a famine account: the horses and pack animals eaten, then whatever could be dug from under the snow, then disease through a camp that could not bury its dead. The details are hostile and the numbers unusable, but the outcome is confirmed from the other side — the Arabic tradition also records the army destroyed by hunger and cold rather than by fighting.
Sulayman died in the autumn of 717 and was succeeded by Umar II, who did not abandon the siege but did not reinforce it adequately either. The army spent the winter waiting for a relief that arrived too late and in the wrong form.
In spring 718 two fleets arrived from Egypt and North Africa with supplies and reinforcements, and they were manned largely by Egyptian Christians. Their crews deserted to the Byzantines in large numbers, taking their ships with them and reporting where the rest were anchored.
The imperial fleet went out and burned them. Constantinople had been supplied by Egypt for centuries before the conquest, and the Umayyad fleet was crewed by the same population — the caliphate had built its navy from the maritime resources of the provinces it had taken, and in 718 that dependence turned on it.
A relieving army marching overland through Anatolia was ambushed and destroyed near Nicomedia in the same season. After that there was no prospect of the siege succeeding and no safe way to maintain it.