The Heraclian Dynasty (610-711) presided over the Byzantine Empire and saw period of cataclysmic events, a watershed in the history of the Empire and the world in general. At the beginning of the dynasty, the Empire was still recognizable as the Eastern Roman Empire, dominating the Mediterranean and harboring a prosperous Late Antiquity urban civilization. This world was shattered by successive invasions, which resulted in extensive territorial losses and financial collapse, plagues that depopulated the cities, while religious controversies and rebellions further weakened the Empire. By the dynasty's end, a very different state had emerged: medieval Byzantium, a chiefly agrarian, military-dominated society which was engaged in a life-and-death struggle with the Muslim Caliphate. However, this state was also far more homogeneous, being reduced to its mostly Greek-speaking and firmly Chalcedonian core territories, which enabled it to weather the storms and enter a period of stability under the successor Isaurian Dynasty.
The Heraclian dynasty was named after the general Heraclius the Younger, who, in 610, sailed from Carthage, overthrew the usurper Phokas, and was crowned Emperor. At the time, the Empire was embroiled in a war with the Sassanid Persian Empire, which in the next decade conquered the Empire's eastern provinces. After a long and exhausting struggle, Heraclius managed to defeat the Persians and restore the Empire, only to lose these provinces again shortly after to the sudden eruption of the Muslim conquests. His successors struggled to contain the Arab tide, to no avail. The Levant and North Africa were lost, while in 674–678, a large Arab army besieged Constantinople itself. Nevertheless, the state survived, and the establishment of the Theme system allowed the imperial heartland, Asia Minor, to be retained. Under Justinian II and Tiberius III the imperial frontier in the East was somewhat stabilized, although incursions continued on both sides. The latter 7th century also saw the first conflicts with the Bulgars, and the establishment of a Bulgarian state in formerly Byzantine lands south of the Danube, which would be the Empire's chief antagonist in the West until the 11th century.