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The Byzantine Empire, c. 867 AD.
The shrinking borders
After Maurice's murder by Phocas, Khosrau used the pretext to reconquer the Roman province of Mesopotamia. Phocas, an unpopular ruler who was invariably described in Byzantine sources as a "tyrant", was the target of a number of senate-led plots. He was eventually deposed in 610 by Heraclius, who sailed to Constantinople from Carthage with an icon affixed to the prow of his ship. Following the ascension of Heraclius, the Sassanid advance pushed deep into Asia Minor, also occupying Damascus and Jerusalem and removing the True Cross to Ctesiphon. The counter-offensive of Heraclius took on the character of a holy war, and an acheiropoietos image of Christ was carried as a military standard. (Similarly, when Constantinople was saved from an Avar siege in 626, the victory was attributed to the icons of the Virgin which were led in procession by Patriarch Sergius about the walls of the city). The main Sassanid force was destroyed at Nineveh in 627, and in 629 Heraclius restored the True Cross to Jerusalem in a majestic ceremony. The war had exhausted both the Byzantine and Sassanid Empire, and left them extremely vulnerable to the Arab Muslim forces which emerged in the following years. The Romans suffered a crushing defeat by the Arabs at the Battle of Yarmuk in 636, and Ctesiphon fell in 634.
The Arabs, now firmly in control of Syria and the Levant, sent frequent raiding parties deep into Anatolia, and between 674 and 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 Ummayyad Caliphate. 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, as the city lost the free grain shipments in 618 after the loss of Egypt to the Persians (province regained in 629, but lost to Arabs invaders in 642). . The void left by the disappearance of the old semi-autonomous civic institutions was filled by the theme system, which entailed the division of Anatolia into "provinces" occupied by distinct armies which assumed civil authority