No spread of Islam eastward. That would mean no Islamic Republic in Iran or Taliban in Afganistan.
That's a bit like saying "Rome loses the Punic Wars, so the Italian rigisiorgensimo won't happen".
How big a victory do the Persians win? Does Rostam manage to stay alive? What condition are the Persian and Arab armies following the battle? Depending on the answers to these question among others makes a huge difference. A Pyrrhic victory for the Persians in which Rostam still dies and the Arabs manage to kill 20,000 to 30,000 of the Persians likely changes very little. On the other hand a decisive victory with Rostam annihilating the Arab forces while still having an army intact enough to stay in the field like sees him linking up with Byzantines to try and smash Khalid’s forces. Long story short I would guestimate if the Caliphate doesn’t manage to firmly occupy Egypt and or Mesopotamia by say 650 it probably gets pushed back into Arabia for the foreseeable future afterwards.
How big a victory do the Persians win? Does Rostam manage to stay alive? What condition are the Persian and Arab armies following the battle? Depending on the answers to these question among others makes a huge difference. A Pyrrhic victory for the Persians in which Rostam still dies and the Arabs manage to kill 20,000 to 30,000 of the Persians likely changes very little. On the other hand a decisive victory with Rostam annihilating the Arab forces while still having an army intact enough to stay in the field like sees him linking up with Byzantines to try and smash Khalid’s forces. Long story short I would guestimate if the Caliphate doesn’t manage to firmly occupy Egypt and or Mesopotamia by say 650 it probably gets pushed back into Arabia for the foreseeable future afterwards.
Islam still spreads to the Indian subcontinent, but nowhere near to the same extent. The Turkic tribes convert to Zoroastrianism, western Christendom is stronger, the Persians eventually take revenge against the Arabs by subduing and persecuting them, Islam remains a fringe religion existing only in the Arabian Peninsula, the Horn of Africa and perhaps the Swahili coast.
What do you mean by spread? If you mean some small conversions in , Malabar Coast and such then I agree. If you mean a vast amount if conversions I think you might be wrong.
To the OP, Islam might still have a chance to take Egypt, but I doubt it will spread for a while after that. Who knows, sometime down the line the Caliphaten may attempt to attack the Persian Empire again.
How much do we actually know about this battle? Is al-Qādisiyyah actually mentioned in any contemporary sources, or is it one of those that are lionised by much later Islamic ones?
My research pretty much says the same thing.Well, it's certainly an important battle.
Sources are essentially Islamic ones of course, and Persian forces may have been exaggerated (both in order to point out divine help is a thing, and to glorify Arab conquerors) but details are plausible up to a certain point, and the social consequences (as in re-"arabisation" of Mesopotamia) partially based on legitimacy from the battle exist enough importantly to not reject these.
Indeed. I feel that it is possible to have been won by a Sassanid state that was in a better position then the Post-Byzantine War, Post-Multiple Civil War Sassanid state that the Arabs faced in OTL.
If perhaps the last war had never really gotten out of the usual stalemate between the two powers and after a few years of back and forth the two had called for peace then Khosrau could have likely met the Rashudin threat rather well.
If Islam fails to take off by smashing the Greeks
The Byzantines were Greeks. They spoke Greek, and were ethnically mostly Greeks. Rome had relied on the Greeks to run things in the east for centuries. That doesn't mean that they weren't also Roman.Dear me!
Dear me!