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So the Roman-Sassanid wars continued on till a hard fought Roman victory that was soon undone by the Islamic conquests.
What if the Sassanids had triumphed decisively?
In essence becoming the Achaemenids come again?
Perhaps driving Rome from Western Asia entirely(or strips on the coast of anatolia?
How would a Sassanid empire that controlled all the Middle East and Egypt up to bythnia fare against the rashidun caliphate?