<P> Heraclius did succeed in establishing a dynasty, and his descendants held onto the throne, with some interruption, until 711 . Their reigns were marked both by major external threats, from the west and the east, which reduced the territory of the empire to a fraction of its 6th - century extent, and by significant internal turmoil and cultural transformation . </P> <P> The Arabs, now firmly in control of Syria and the Levant, sent frequent raiding parties deep into Asia Minor, and in 674--678 laid siege to Constantinople itself . The Arab fleet was finally repulsed through the use of Greek fire, and a thirty - years' truce was signed between the Empire and the Umayyad Caliphate . However, the Anatolian raids continued unabated, and accelerated the demise of classical urban culture, with the inhabitants of many cities either refortifying much smaller areas within the old city walls, or relocating entirely to nearby fortresses . Constantinople itself dropped substantially in size, from 500,000 inhabitants to just 40,000--70,000, and, like other urban centers, it was partly ruralised . The city also lost the free grain shipments in 618, after Egypt fell first to the Persians and then to the Arabs, and public wheat distribution ceased . The void left by the disappearance of the old semi-autonomous civic institutions was filled by the theme system, which entailed the division of Asia Minor into "provinces" occupied by distinct armies which assumed civil authority and answered directly to the imperial administration . This system may have had its roots in certain ad hoc measures taken by Heraclius, but over the course of the 7th century it developed into an entirely new system of imperial governance . </P> <P> The withdrawal of massive amounts of troops from the Balkans to combat the Persians and then the Arabs in the east opened the door for the gradual southward expansion of Slavic peoples into the peninsula, and, as in Anatolia, many cities shrank to small fortified settlements . In the 670s the Bulgars were pushed south of the Danube by the arrival of the Khazars, and in 680 Byzantine forces which had been sent to disperse these new settlements were defeated . In the next year Constantine IV signed a treaty with the Bulgar khan Asparukh, and the new Bulgarian state assumed sovereignty over a number of Slavic tribes which had previously, at least in name, recognized Byzantine rule . In 687--688, the emperor Justinian II led an expedition against the Slavs and Bulgars which made significant gains, although the fact that he had to fight his way from Thrace to Macedonia demonstrates the degree to which Byzantine power in the north Balkans had declined . </P> <P> The one Byzantine city that remained relatively unaffected, despite a significant drop in population and at least two outbreaks of the plague, was Constantinople . However, the imperial capital was marked by its own variety of conflict, both political and religious . Constans II continued the monothelite policy of his grandfather, Heraclius, meeting with significant opposition from laity and clergy alike . The most vocal opponents, Maximus the Confessor and Pope Martin I were arrested, brought to Constantinople, tried, tortured, and exiled . Constans seems to have become immensely unpopular in the capital, and moved his residence to Syracuse, Sicily, where he was ultimately murdered by a member of his court . The Senate experienced a revival in importance in the seventh century and clashed with the emperors on numerous occasions . The final Heraclian emperor, Justinian II, attempted to break the power of the urban aristocracy through severe taxation and the appointment of "outsiders" to administrative posts . He was driven from power in 695, and took shelter first with the Khazars and then with the Bulgars . In 705 he returned to Constantinople with the armies of the Bulgar khan Tervel, retook the throne, and instituted a reign of terror against his enemies . With his final overthrow in 711, supported once more by the urban aristocracy, the Heraclian dynasty came to an end . </P>

Throughout its history the byzantine empire was most threatened by attacks from the