The Persians move into N. Africa after conquering Egypt in 618 AD, during the reign of Chosroes II. The demoralized population offers little resistance and the Persians advance as far as Carthage. This means that the counter offensive against the Persians offered by Hercalius can't happen now. Would the Byzantines survive in eastern Europe? What happens during the Islamic invasion, if the Arabs are confronted by an immense Persian empire?