The Byzantine Empire under the Isaurian dynasty (or Syrian dynasty) ruled from 717 to 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 favour 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 an attempt at making the Carolingian Empire the successor to the Roman Empire.
Background: Byzantium in the 7th century
The Heraclian dynasty (610–695 and 705–711) faced heavy challenges. After successfully overcoming the Sassanid Persians, the Emperor Heraclius (r. 610–641) and his exhausted realm were faced with the sudden onset of the Muslim expansion from Arabia into the Levant.
Following the Muslim conquest of Syria, the rich province of Egypt, the Empire's chief source of grain and tax revenue, had fallen to the Arabs. The Byzantines also faced Arab attacks through Libya against the Exarchate of Africa, against Cilicia, which controlled the southern passes into Asia Minor, now the Empire's last major contiguous territory, and against the Armenian Highland, the Empire's chief source of manpower and a vital buffer between the now Arab-dominated Syrian Desert region and the northeastern passage into Asia Minor. These three areas would be the main fields of Byzantine-Arab contention during the next half-century. The Arabs continued to make headway, most notably constructing a navy that successfully challenged Byzantine supremacy in the Mediterranean. The outbreak of the Muslim civil war in 656 bought the Byzantines time, and emperor Constans II (r. 641–668) reinforced his position in the Balkans and Italy. His successor, Constantine IV (r. 668–685), was able to beat off the First Arab Siege of Constantinople (674–678), and in its aftermath move into the counteroffensive, securing Asia Minor, recovering Cilicia and forcing the Caliphate to pay tribute. At the same time however, he was defeated by the Bulgar khan Asparukh, and was forced to accept his people's settlement in Byzantine lands south of the Danube. With the first deposition of Constantine IV's son and heir Justinian II in 695 began a period of troubles that lasted almost a quarter-century and brought a succession of disasters that nearly brought about the downfall of the Byzantine state. Carthage finally fell in 697 and a Byzantine recovery attempt defeated next year. Cilicia was conquered by the Arabs and turned into a base for raiding expeditions that penetrated deep into Asia Minor, sacking its forts and cities, while the Caucasus brought under firm Muslim control. Finally, the Umayyad caliph Sulayman ibn Abd al-Malik (r. 715–717) began preparing another huge expedition to conquer Constantinople.