The Byzantine Empire was ruled by the Isaurian or Syrian Dynasty from 717-802. The Isaurian emperors were successful in defending and consolidating the empire against the Caliphate after the onslaught of the early Muslim conquests, but were less successful in Europe, where they suffered setbacks against the Bulgars, had to give up the Exarchate of Ravenna, and lost influence over Italy and the Papacy to the growing power of the Franks.

The Isaurian Dynasty is chiefly associated with Byzantine Iconoclasm, an attempt to restore divine favor by purifying the Christian faith from excessive adoration of icons, which resulted in considerable internal turmoil.

By the end of the Isaurian Dynasty in 802, the Byzantines were continuing to fight the Arabs and the Bulgars for their very existence, with matters made more complicated when Pope Leo III crowned Charlemagne Imperator Romanorum (“Emperor of the Romans”), which was seen as making the Carolingian Empire the successor to the Roman Empire, or at least the western half.

Leo III, who would become the founder of the so-called Isaurian Dynasty, was actually born in Germanikeia in northern Syria c. 685; his alleged origin from Isauria derives from a reference in Theophanes the Confessor, which may be a later addition. After being raised to spatharios by Justinian II, he fought the Arabs in Abasgia, and was appointed as strategos of the Anatolics by Anastasios II. Following the latter’s fall in 716, Leo allied himself with Artabasdos, the general of the Armeniacs, and was proclaimed emperor while two Arab armies campaigned in Asia Minor. Leo averted an attack by Maslamah through clever negotiations, in which he promised to recognize the Caliph’s suzerainty. However, on March 25, 717, he entered Constantinople and deposed Theodosios.

Leo III’s Rule

Having preserved the empire from extinction by the Arabs, Leo proceeded to consolidate its administration, which in the previous years of anarchy had become completely disorganized. In 718, he suppressed a rebellion in Sicily and in 719 did the same on behalf of the deposed Emperor Anastasios II.

Leo secured the empire’s frontiers by inviting Slavic settlers into the depopulated districts, and by restoring the army to efficiency; when the Umayyad Caliphate renewed their invasions in 726 and 739, as part of the campaigns of Hisham ibn Abd al-Malik, the Arab forces were decisively beaten, particularly at Akroinon in 740. His military efforts were supplemented by his alliances with the Khazars and the Georgians.

Leo undertook a set of civil reforms, including the abolition of the system of prepaying taxes, which had weighed heavily upon the wealthier proprietors; the elevation of the serfs into a class of free tenants; and the remodeling of family, maritime law, and criminal law, notably substituting mutilation for the death penalty in many cases. The new measures, which were embodied in a new code called the Ecloga (Selection), published in 726, met with some opposition on the part of the nobles and higher clergy. The emperor also undertook some reorganization of the theme structure by creating new themata in the Aegean region.

The Siege of Constantinople

The Second Arab siege of Constantinople in 717-718 was a combined land and sea offensive by the Muslim Arabs of the Umayyad Caliphate against the capital city of the Byzantine Empire, Constantinople. 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, tricked them and secured the Byzantine throne for himself.

After wintering in the western coastlands of Asia Minor, the Arab army crossed into Thrace in early summer 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. 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 August 15, 718. On its return journey, the Arab fleet was almost completely destroyed by natural disasters and Byzantine attacks.

The Arab failure was 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 siege’s failure had wide-ranging repercussions. The rescue of Constantinople ensured the continued survival of Byzantium, 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. The Byzantine capital’s survival preserved the empire as a bulwark against Islamic expansion into Europe until the 15th century, when it fell to the Ottoman Turks. Along with the Battle of Tours in 732, the successful defense of Constantinople has been seen as instrumental in stopping Muslim expansion into Europe.

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