After wintering in the western coastlands of Asia Minor, the Arab army crossed into Thrace in the early summer of 717 and built siege lines to blockade the city, which was protected by the massive Theodosian Walls.

The Arab fleet, which accompanied the land army and was meant to complete the city's blockade by sea, was 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, and an additional army sent overland through Asia Minor was ambushed and defeated.

[7] In this, the Arabs were aided by the prolonged period of internal instability that followed the first deposition of Justinian II in 695, in which the Byzantine throne changed hands seven times in violent coups.

Emperor Anastasius II (r. 713–715) sent an embassy to Damascus under the patrician and urban prefect, Daniel of Sinope, ostensibly in order to plea for peace, but in reality to spy on the Arabs.

At Rhodes, however, the Byzantine fleet, encouraged by the soldiers of the Opsician Theme, rebelled, killed their commander John the Deacon and sailed north to Adramyttium.

[20] The accession of Theodosius, who from the sources comes across as both unwilling and incapable, as a puppet emperor of the Opsicians provoked the reaction of the other themes, especially the Anatolics and the Armeniacs under their respective strategoi ('generals') Leo the Isaurian and Artabasdos.

Amorium had been left defenceless in the turmoil of the civil war and would have easily fallen, but the Arabs chose to bolster Leo's position as a counterweight to Theodosius.

[24][25] Leo's success at Amorium was fortunately timed, since Maslama with the main Arab army had in the meantime crossed the Taurus Mountains and was marching straight for the city.

In addition, as the Arab general had not received news of Leo's double-dealing, he did not devastate the territories he marched through—the Armeniac and Anatolic themes, whose governors he still believed to be his allies.

[26] On meeting up with Sulayman's retreating army and learning what had transpired, Maslama changed direction: he attacked Akroinon and from there marched to the western coastlands to spend the winter.

The supply train alone is said to have numbered 12,000 men, 6,000 camels and 6,000 donkeys, while according to the 13th-century historian Bar Hebraeus, the troops included 30,000 volunteers (mutawa) for the Holy War (jihad).

[30] Little is known on the detailed composition of the Arab force, but it appears that it mostly consisted of, and was led by, Syrians and Jazirans of the elite ahl al-Sham ('People of Syria'), the main pillar of the Umayyad regime and veterans of the struggle against Byzantium.

Aside from Anastasius II's preparations (which might have been neglected following his deposition),[34] the Byzantines could count on the assistance of the Bulgar ruler Tervel, with whom Leo concluded a treaty that possibly included alliance against the Arabs.

Two days later, Sulayman led his fleet into the Bosphorus and the various squadrons began anchoring by the European and Asian suburbs of the city: one part sailed south of Chalcedon to the harbours of Eutropios and Anthemios to watch over the southern entrance of the Bosporus, while the rest of the fleet sailed into the strait, passed by Constantinople and began making landfall on the coasts between Galata and Kleidion, cutting the Byzantine capital's communication with the Black Sea.

The victory encouraged the Byzantines and dejected the Arabs, who, according to Theophanes, had originally intended to sail to the sea walls during the night and try to scale them using the ships' steering paddles.

As the supplies in the Arab camp ran out, a terrible famine broke out: the soldiers ate their horses, camels, and other livestock, and the bark, leaves and roots of trees.

[41] Consequently, the Arab army was ravaged by epidemics; with great exaggeration, the Lombard historian Paul the Deacon put the number of their dead of hunger and disease at 300,000.

When the new fleets arrived in the Sea of Marmara, they kept their distance from the Byzantines and anchored on the Asian shore, the Egyptians in the Gulf of Nicomedia near modern Tuzla and the Africans south of Chalcedon (at Satyros, Bryas and Kartalimen).

[43] On land too the Byzantines were victorious: their troops managed to ambush the advancing Arab army under a commander named Mardasan and destroy it in the hills around Sophon, south of Nicomedia.

Michael the Syrian on the other hand mentions that the Bulgars participated in the siege from the beginning, with attacks against the Arabs as they marched through Thrace towards Constantinople, and subsequently on their encampment.

[47] Arab sources claim that altogether 150,000 Muslims perished during the campaign, a figure which, according to the Byzantinist John Haldon, "while certainly inflated, is nevertheless indicative of the enormity of the disaster in medieval eyes".

[49] The blow to the Caliphate's might was severe, and although the land army did not suffer losses in the same degree as the fleet, Umar is recorded as contemplating withdrawing from the recent conquests of Hispania and Transoxiana, as well as a complete evacuation of Cilicia and other Byzantine territories that the Arabs had seized over the previous years.

[51] Leo also restored control over Sicily, where news of the Arab siege of Constantinople and expectations of the city's fall had prompted the local governor to declare an emperor of his own, Basil Onomagoulos.

[33] The siege represented a final effort by the Caliphate to "cut off the head" of the Byzantine Empire, after which the remaining provinces, especially in Asia Minor, would be easy to capture.

[54] The reasons for the Arab failure were chiefly logistical, as they were operating too far from their Syrian bases, but the superiority of the Byzantine navy through the use of Greek fire, the strength of Constantinople's fortifications, and the skill of Leo III in deception and negotiations also played important roles.

The Muslim goal of conquest of Constantinople was effectively abandoned, and the frontier between the two empires stabilized along the line of the Taurus and Antitaurus Mountains, over which both sides continued to launch regular raids and counter-raids.

Bury called 718 "an ecumenical date", while the Greek historian Spyridon Lambros likened the siege to the Battle of Marathon and Leo III to Miltiades.

In legend, the defeat was transformed into a victory: Maslama departed only after symbolically entering the Byzantine capital on his horse accompanied by thirty riders, where Leo received him with honour and led him to the Hagia Sophia.

[70] ^ b: According to the historian Hugh N. Kennedy, based on the numbers found in the contemporary army registers (diwans), the total manpower available to the Umayyad Caliphate c. 700 ranged between 250,000 and 300,000 men, spread throughout the various provinces.