In 717–718, Constantinople, the capital of the Byzantine Empire, was besieged by the Muslim Arabs of the Umayyad Caliphate. The campaign marked the culmination of twenty years of attacks and progressive Arab occupation of the Byzantine borderlands, while Byzantine strength was sapped by prolonged internal turmoil. In 716, after years of preparations, the Arabs, led by Maslama ibn Abd al-Malik, invaded Byzantine Asia Minor. The Arabs initially hoped to exploit Byzantine civil strife and made common cause with the general Leo III the Isaurian, who had risen up against Emperor Theodosius III. Leo, however, deceived them and secured the Byzantine throne for himself.
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 partly neutralized soon after its arrival by the Byzantine navy through the use of Greek fire. This allowed Constantinople to be resupplied by sea, while the Arab army was crippled by famine and disease during the unusually hard winter that followed. 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. Coupled with attacks by the Bulgars on their rear, the Arabs were forced to lift the siege on 15 August 718. On its return journey, the Arab fleet was almost completely destroyed by natural disasters.
The siege's failure had wide-ranging repercussions. The rescue of Constantinople ensured the continued survival of Byzantium and marked the end of a century of constant war, territorial losses and internal strife, while the Caliphate's strategic outlook was altered: although regular attacks on Byzantine territories continued, the goal of outright conquest was abandoned. Historians consider the siege to be one of history's most important battles, as its failure postponed the Muslim advance into Southeastern Europe for centuries.
Following the first Arab siege of Constantinople (674–678), the Arabs and Byzantines experienced a period of peace. After 680, the Umayyad Caliphate was in the throes of the Second Muslim Civil War, and the consequent Byzantine ascendancy in the East enabled the emperors to extract huge amounts of tribute from the Umayyad government in Damascus. In 692, as the Umayyads emerged as victors from their civil war, Emperor Justinian II (r. 685–695, 705–711) resumed hostilities with the Caliphate. The result was a series of Arab victories that led to the loss of Byzantine control over Armenia and the Caucasian principalities and a gradual encroachment upon the Byzantine borderlands. Year by year, the Caliphate's generals, usually members of the Umayyad family, launched raids into Byzantine territory and captured fortresses and towns. After 712, the Byzantine defensive system began to show signs of collapse: Arab raids penetrated further and further into Asia Minor, border fortresses were repeatedly attacked and sacked, and references to Byzantine reaction in the sources become more and more scarce. 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. In the words of the Byzantinist Warren Treadgold, "the Arab attacks would in any case have intensified after the end of their own civil war.” With far more men, land, and wealth than Byzantium, the Arabs had begun to concentrate all their strength against it. Now they threatened to extinguish the empire entirely by capturing its capital."
The information available on the siege comes from sources composed in later dates, which are often mutually contradictory. The main Byzantine source is the extensive and detailed account of the Chronicle of Theophanes the Confessor (760–817) and secondarily the brief account in the Breviarium of Patriarch Nikephoros I of Constantinople (died 828), which shows small differences, mainly chronological, from Theophanes's version. For the events of the siege, both authors appear to have used a primary account composed during the reign of Leo III the Isaurian (r. 717–741) which therefore contains a favourable depiction of the latter, while Theophanes apparently relies on an unknown biography of Leo (ignored by Nikephoros) for the events of 716. The 8th-century chronicler Theophilus of Edessa records the years leading up to the siege and the siege itself in some detail, paying particular attention to the diplomacy between Maslama and Leo III. The Arab sources, mainly the 11th-century Kitab al-'Uyun and the more concise narrative in the History of the Prophets and Kings by al-Tabari (838–923), rely on primary accounts by early 9th-century Arab writers, but are more confused and contain several legendary elements. The Syriac language accounts are based on Agapius of Hierapolis (died 942), who likely drew from the same primary source as Theophanes, but are far briefer.
Opening stages of the campaign
The Arab successes opened the way for a second assault on Constantinople, an undertaking already initiated under Caliph al-Walid I (r. 705–715). Following his death, his brother and successor Sulayman (r. 715–717) took up the project with increased vigour, according to Arab accounts because of a prophecy that a Caliph bearing the name of a prophet would capture Constantinople; Sulayman (Solomon) was the only member of the Umayyad family to bear such a name. According to Syriac sources, the new Caliph swore "to not stop fighting against Constantinople before having exhausted the country of the Arabs or to have taken the city". The Umayyad forces began assembling at the plain of Dabiq north of Aleppo, under the direct supervision of the Caliph. As Sulayman was too sick to campaign himself, however, he entrusted command to his brother Maslama ibn Abd al-Malik. The operation against Constantinople came at a time when the Umayyad empire was undergoing a period of continuous expansion to the east and west. Muslim armies advanced into Transoxiana, India, and the Visigothic Kingdom of Hispania.
Arab preparations, especially the construction of a large fleet, did not go unnoticed by the worried Byzantines. 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. Anastasius, in turn, began to prepare for the inevitable siege: the fortifications of Constantinople were repaired and equipped with ample artillery (catapults and other siege weapons), while food stores were brought into the city. In addition, those inhabitants who could not stockpile food for at least three years were evacuated. Anastasius strengthened his navy and in early 715 dispatched it against the Arab fleet that had come to Phoenix—usually identified with modern Finike in Lycia, it may also be modern Fenaket across Rhodes, or perhaps Phoenicia (modern Lebanon), famed for its cedar forests—to collect timber for their ships. 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. There, they acclaimed a reluctant tax collector, Theodosius, as emperor. Anastasius crossed into Bithynia in the Opsician Theme to confront the rebellion, but the rebel fleet sailed on to Chrysopolis. From there, it launched attacks against Constantinople, until, in late summer, sympathizers within the capital opened its gates to them. Anastasius held out at Nicaea for several months, finally agreeing to resign and retire as a monk. 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.