After the Byzantines killed Maurice, Khosrow II began a war in 602 against the Byzantines. Khosrow II's forces captured much of the Byzantine Empire's territories, earning the king the epithet "the Victorious". A siege of the Byzantine capital of Constantinople in 626 was unsuccessful, and Heraclius, now allied with Turks, started a successful risky counterattack deep into Persia's heartland. Supported by the feudal families of the empire, Khosrow II's imprisoned son Sheroe (Kavad II) imprisoned and killed Khosrow II. This led to a civil war and interregnum in the empire and the reversal of all Sasanian gains in the war against the Byzantines.