I am not an expert on this era, but imagine a world where the Sasanian Empire of Persia had defeated the Byzantine Empire at some point during the later stages of their final war from 602 to 628 - that is, at some point in the 620s. I am sure that there are any number of points of divergence that could have made this a reality, but the important thing is that the Persians end the conflict firmly in control of the Levant, Egypt, and much of eastern Anatolia (their allies, the Avars, may make significant gains in southeastern Europe depending on the point of divergence). The Byzantines are reduced to their territories in coastal Greece, western Anatolia, Crimea, and the Exarchates of Africa and Ravenna. So just as the Byzantines reigned supreme in the Eastern Mediterranean for a brief moment after the war in our timeline, the Sasanians are similarly dominant here. But we all know what comes next. The early Muslims are still going to be consolidating power in Arabia while the two aforementioned empires are fighting, which in our world laid the groundwork for their conquests in the wider world. How would their campaigns in the Levant and Egypt have gone differently had the Persians controlled those regions? I am sure that the Sassanian Empire would be overextended here, especially after such a costly war, which may make their new gains vulnerable. Then again, much of the success of the Rashidun forces in these wars came down to sheer luck. Would the Muslim conquests of these regions have been more or less likely than in our timeline? Assuming that the Rashidun Caliphate is still successful in the Levant and Egypt, are they also still likely to conquer the entirety of Persia? I am very interested in how the Byzantine Empire might react to this situation. As they and the Rashidun Caliphate do not share a border, and indeed have a great deal of Sassanian-controlled territory separating them, might they even form an alliance in order to fight their common enemy? It would certainly give the Byzantine Empire an opportunity to regain some of the territory that they had lost very recently, though the exact boundary between their lands and the new Rashidun Caliphate will be a point of contention. Even more interestingly, what does this mean for the relationship between Islam and Christianity in this world? If I am not mistaken, I imagine that much of the early Christian hostility toward Islam ultimately stemmed from the fact that the major Christian power of that day had fought bitterly with the early Muslims. If that is not the case here, and if the two are even allies at this critical juncture, I imagine that the two faiths could regard each other profoundly differently in this world than they do in ours. Finally, what would be the other long-term implications of such a scenario?