Siege of Constantinople (717–718)
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, 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 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."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. Following his death, his brother and successor Sulayman 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 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 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, 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, 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 Leo the Isaurian and Artabasdos.
In these conditions of near-civil war, the Arabs began their carefully prepared advance. In September 715, the vanguard, under general Sulayman ibn Mu'ad, marched over Cilicia into Asia Minor, taking the strategic fortress of Loulon on its way. They wintered at Afik, an unidentified location near the western exit of the Cilician Gates. In early 716, Sulayman's army continued into central Asia Minor. The Umayyad fleet under Umar ibn Hubayra cruised along the Cilician coast, while Maslama ibn Abd al-Malik awaited developments with the main army in Syria.
The Arabs hoped that the disunity among the Byzantines would play to their advantage. Maslama had already established contact with Leo the Isaurian. French scholar Rodolphe Guilland theorized that Leo offered to become a vassal of the Caliphate, although the Byzantine general intended to use the Arabs for his own purposes. In turn, Maslama supported Leo hoping to maximize confusion and weaken the Empire, easing his own task of taking Constantinople.
Sulayman's first objective was the strategically important fortress of Amorium, which the Arabs intended to use as a base the following winter. 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. They offered the city terms of surrender if its inhabitants would acknowledge Leo as emperor. The fortress capitulated, but still did not open its gates to the Arabs. Leo came to the vicinity with a handful of soldiers and executed a series of ruses and negotiations to garrison 800 men in the town. The Arab army, thwarted in its objective and with supplies running low, withdrew. Leo escaped to Pisidia and, in summer, supported by Artabasdos, was proclaimed and crowned as Byzantine emperor, openly challenging Theodosius.
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. 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. On his way, he sacked Sardis and Pergamon. The Arab fleet wintered in Cilicia. Leo, in the meantime, began his own march on Constantinople. He captured Nicomedia, where he found and captured, among other officials, Theodosius's son, and then marched to Chrysopolis. In spring 717, after short negotiations, he secured Theodosius's resignation and his recognition as emperor, entering the capital on 25 March. Theodosius and his son were allowed to retire to a monastery as monks, while Artabasdos was promoted to the position of and received the hand of Leo's daughter, Anna.
Opposing forces
From the outset, the Arabs prepared for a major assault on Constantinople. The late 8th-century Syriac Zuqnin Chronicle reports that the Arabs were "innumerable", while the 12th-century Syriac chronicler Michael the Syrian mentions a much-inflated 200,000 men and 5,000 ships. The 10th-century Arab writer al-Mas'udi mentions 120,000 troops, and the account of Theophanes the Confessor 1,800 ships. Supplies for several years were hoarded, and siege engines and incendiary materials were stockpiled. 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 for the Holy War. The Byzantines' strength is entirely unknown, but Constantinople's defenders likely did not number over 15,000 men, given both the exhaustion of the Byzantine Empire's manpower and the limitations imposed by the need to maintain and feed such a force.Whatever the true numbers, the attackers were considerably more numerous than the defenders; according to Treadgold, the Arab host may have outnumbered the entire Byzantine army. 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 , the main pillar of the Umayyad regime and veterans of the struggle against Byzantium. Alongside Maslama, Umar ibn Hubayra, Sulayman ibn Mu'ad, and Bakhtari ibn al-Hasan are mentioned as his lieutenants by Theophanes and Agapius of Hierapolis, while the later replaces Bakhtari with Abdallah al-Battal.
Although the siege consumed a large part of the Caliphate's manpower and resources, it was still capable of launching raids against the Byzantine frontier in eastern Asia Minor during the siege's duration: in 717, Caliph Sulayman's son Daud captured a fortress near Melitene and in 718 Amr ibn Qais raided the frontier. On the Byzantine side, the numbers are unknown. Aside from Anastasius II's preparations, 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.