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The Battle of al-Qādisiyyah was one of the most decisive battles between the expanding Muslim Arabs and the declining Sassanid Empire.
After the battle the Sassanid rule in what is modern day Iraq was pretty much doomed.
So what do you all think the long and short term results of a major Sassanid victory be? At the very least if the Empire holds out against the Arabs then Zoroastrianism could still be a major religion even today.