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.
Background
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."