Khosrow II was overthrown in 628 and was replaced by the rather Monophysite Byzantine Emperor Maurice's grandson Kavad II in Old Medina who appointed Heraclius regent over his son when he died.
===The Critical Year===
Understanding the chronology of what happened next is essential.
The Monophysite Church immediately broke communion with Heraclius and established a Maphrian of all the East to oppose the Nestorian Catholicos. This may have been to appease their congregations who were no doubt furious about being united to the Nestorians.
At the same time the Pahlavis had different ideas and rejected Heraclius's regency over Kavodh's young son Ardashir whom they established on the throne instead triggering the chain of events which soon led to Arab supremacy in the region. There was nothing supernatural about it.