Sorry I'm posting so frequently I have a lot of ideas and questions in my head about Alternate History.
So, how could the Sassanid Empire hold off against the Rashidun Caliphate?
Is it even possible?
Is there anyway that in our modern world we could see a Zoroastrian Persia instead of an Islamic Iran (Persia)?
Any other comments would be great as well. Thank you to those who comment, as usual.
The Sassanids got really unlucky against the Arabs. It is difficult for them to stem the Arab tide in Mesopotamia (the country was really wrecked by Khosrau II). However, it isn't implausible for the Persians to retain some of Mesopotamia. It is absolutely plausible for the Persians to lose only Mesopotamia, but retain the rest of the empire.
How things play out in the long run is more difficult. It is quite possible that, just like the Romans and the Indians, the Persians resist the initial attempts at conquest, but fall to a later set of Muslim conquerors. It is also quite possible that Persia is converted to Islam by missionaries, rather than by conquerors.
The effects on Persian culture and on Islam itself would be fairly huge - the evolution of the early faith is changed, there may be no Islamic agricultural revolution, there may be no Islamic scientific revolution (Persians as well as trade across Persia and Central Asia were key to how both of the latter unfolded in OTL). Likely, the Persian literary flowering that followed the conquest is delayed for some centuries, since the nobility of the country can continue to act in their traditional military capacity - in OTL, the Arabs kept the Persians out of military work for some time, pushing talent into the civil service, which encouraged the wealthy classes to develop the literacy and numeracy of their sons - which meant those same sons started writing poetry and writing histories of their grand culture (since that culture was under thread from Arab domination).
I suspect that absent the conquests, while the decline of Zoroastrianism would be slower, it would continue. The religion had already been losing ground to Christianity before the Arab conquests, and most accounts I've read have thought that Christianity was equal in popularity with Zoroastrianism by the time of the conquest, with each accounting for about 40% of Persia's population. The remaining third was mostly accounted for by Buddhists and Jews.
So even if Persia continues to resist Islam (both in missionary and conquering forms) I doubt it would be a Zoroastrian majority country. More likely it is, like India, a bewildering mix of faiths.
fasquardon