Let's say that after Phocas the Tyrant takes power in 602, Khusrau II has a dream from Zoroaster commanding him to recognize Phocas and any successors, whether by force or inheritance, as legitimate. This means that Khusrau II does not invade the Empire shortly thereafter, and even if Heraclius the Younger topples Phocas for losing the Balkans, that Persia still doesn't invade (at least, not for that reason). So, what happens when the Caliphate invades from the south with the Persians and the Romans at, more or less, full strength?