The Second Arab Siege of Constantinople
After the failed Arab Siege of Constantinople (674-678), which the traditional view of events and even actual occurrence has been questioned, the Arabs launched another combined land and sea effort to take the capital of the Byzantine Empire later. This later effort would lead to the Second Arab Siege of Constantinople (717-718). An 80,000 strong army led by Maslama ,brother of Caliph Umar II crossed the Bosporus, while a fleet of around 1,800 ships sailed into the Marmara, south of the city. Emperor Leo III was able to use Constantinople's famed walls to his advantage, as it was. Meanwhile, the fleet failed to sail up the Bosporus, as the Greek fleet harassed it, using Greek Fire.
The Siege depicted in the 14th century Bulgarn translation of the Manasses Chroncle
The winter of of 717/718 was very harsh, and the Arab land forces were hit hard, due to shortages of supplies. However, Constantinople was supplied via the Black Sea, therefore the winter did not have an as large of an effect on the Byzantines. An Egyptian fleet of around 400 ships and an African fleet of around 360 ships arrived in the spring time and a fresh army started to march through Asia Minor. Many Christian Egyptians in the Arab fleets began to defect to the Byzantines. Furthermore, successive assaults on Constantinople could still not breach the walls. The Bulgarians who had established friendly relations with the Byzantines a year, came to the aid of Constantinople and in July the Arabs were devastated by a Bulgar attack. Contempraries report at least 30,000 Arabs died in the first Bulgarian attack. The attack was likely devastating due to the combination of starvation, disease and being demoralised due to multiple failed assaults on the city.
In August 718 the Arabs were forced to retreat in the face of the Bulgarian onslaught and the lack of successes against the city. Part of the Arab army tried to retreat through Anatolia, whereas the other half attempted to retreat on the remaining Arab vessels. A devastating storm hit the Arab fleet on the way back. All but 5 of the ships were destroyed and the retreating Arab forces on-board the ships were destroyed.
The failure to take Constantinople was a severe blow to Caliph Umar II and the Umayyad Caliphate's general expansion. If the city had fallen it is possible the Byzantine Empire would have disintegrated, opening Europe to Muslim incursions centuries before the Ottoman Empire and while Europe was in turmoil following the fall of Rome. The blow to the Caliphate's might led to Umar II contemplating an evacuation of recent Arab conquests, such as Transoxiana and Hispania and even abandoning the Arab conquests of some Byzantine territory. Although Umar's advisers persuaded him not to take such drastic actions, most Arab garrisons were withdrawn from Byzantine frontier districts. This battle can be compared to the Battle of Tours, as they both helped in halting Islam's expansion into Europe and in establishing Christian dominance on the continent.