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Initial Persian advantage

During his Balkan campaigns, Emperor Maurice and his family were murdered by Phocas in November 602 after a mutiny. Khosrow II (Chosroes) of the Sasanian Empire had been restored to his throne by Maurice, and they had remained allies until the latter's death. Thereafter, Khosrow seized the opportunity to attack the Byzantine Empire and reconquer Mesopotamia. Khosrow had at his court a man who claimed to be Maurice's son Theodosius, and Khosrow demanded that the Byzantines accept this Theodosius as emperor.

The war initially went the Persians' way, partly because of Phocas's brutal repression and the succession crisis that ensued as the general Heraclius sent his nephew Nicetas to attack Egypt, enabling his son Heraclius the younger to claim the throne in 610. Phocas, an unpopular ruler who is invariably described in historical sources as a " tyrant " (in its original meaning of the word, i.e. illegitimate king by the rules of succession), was eventually deposed by Heraclius, who sailed to Constantinople from Carthage with an icon affixed to the prow of his ship.

By this time, the Persians had conquered Mesopotamia and the Caucasus, and in 611 they overran Syria and entered Anatolia. A major counter-attack led by Heraclius two years later was decisively defeated outside Antioch by Shahrbaraz and Shahin, and the Roman position collapsed; the Persians devastated parts of Asia Minor and captured Chalcedon across from Constantinople on the Bosporus.

Over the following decade the Persians were able to conquer Palestine and Egypt (by mid-621, the whole province was in their hands) and to devastate Anatolia, while the Avars and Slavs took advantage of the situation to overrun the Balkans, bringing the Empire to the brink of destruction. In 613, the Persian army took Damascus with the help of the Jews, seized Jerusalem in 614, damaging the Church of the Holy Sepulchre and capturing the True Cross, and afterwards capturing Egypt in 617 or 618. When the Sasanians reached Chalcedon in 615, it was at this point, according to Sebeos, that Heraclius had agreed to stand down and was about ready to allow the Byzantine Empire to become a Persian client state, even permitting Khosrow II to choose the emperor. In a letter delivered by his ambassadors, Heraclius acknowledged the Persian empire as superior, described himself as Khosrow II's "obedient son, one who is eager to perform the services of your serenity in all things", and even called Khosrow II the "supreme emperor". Khosrow II nevertheless rejected the peace offer, and arrested Heraclius' ambassadors.

With the Persians at the very gate of Constantinople, Heraclius thought of abandoning the city and moving the capital to Carthage, but the powerful church figure Patriarch Sergius convinced him to stay. Safe behind the walls of Constantinople, Heraclius was able to sue for peace in exchange for an annual tribute of a thousand talents of gold, a thousand talents of silver, a thousand silk robes, a thousand horses, and a thousand virgins to the Persian King. The peace allowed him to rebuild the Empire's army by slashing non-military expenditure, devaluing the currency, and melting down, with the backing of Patriarch Sergius, Church treasures to raise the necessary funds to continue the war.