Premise: After losing hold of Mesopotamia, the Sassanids manage to halt the Arab advance past the Zagros mountains. The Zagros mountains are a decently defensible position, although Persia was in disarray at the time and the loss of wealthy Mesopotamia would hardly make rebuilding easier.
The Sassanids(or any successor dynasty) are also highly unlikely to give up on attempts to reconquer Mesopotamia. So I find it most likely that if Islamic forays into the Iranian plateau are thwarted the Persians will eventually(let's say, before the end of the 7th century) recapture Mesopotamia, perhaps during one of the Fitnas.
Even having lost Mesopotamia, the Caliphate still holds Egypt and Syria. No doubt the Persians would consider advancing into Syria... but then, the Byzantines also desired to reclaim Syria. Thus the threeway balance of power- if one of three holds all of Syria, it is has to defend on two fronts against the other two. I'm not sure how stable this dynamic would be, it certainly couldn't remain indefinitely. But if one of the three advances deep into another's territory, it leaves itself deeply exposed to the third- so it might endure for some time.
Aside from Syria, there are also fronts on which only two of the three would be at odds. The Caliphate and Byzantines would be vying over the Maghreb and Mediterranean islands, the Byzantines and Sassanids would likely to come into conflict over Eastern Anatolia/the Caucasus, and there would no doubt be Arab raids into Mesopotamia.
Also, depending on how long the Caliphate holds Mesopotamia before Persia reconquers it, there will be a substantial population of Muslims there. It's possible that the Persians would slaughter/forcefully convert them. But they might instead repeat their strategy with Christians(where they favoured Nestorian Christianity, so as to drive a wedge between Christians living in their territory and Roman Christians). Even IOTL early Islam was faced with schismatic sects like Shia and Kharijites, and conveniently they were most successful in areas that are easily accessible to Persia(Shia in Mesopotamia and the East coast of Arabia, and the Ibadis(Kharijite descendants) in Oman.
This would also have implications for the spread of Islam along Indian ocean trade routes- if the Persians choose to favour Shia and Kharijite factions in Eastern and Southern Arabia, then conversion to Islam due to the influence of Arab traders in Southern India and South East Asia may follow those denominations rather then being Sunni. Or perhaps the split would inhibit the spread of Islam along trade routes entirely?
The Sassanids(or any successor dynasty) are also highly unlikely to give up on attempts to reconquer Mesopotamia. So I find it most likely that if Islamic forays into the Iranian plateau are thwarted the Persians will eventually(let's say, before the end of the 7th century) recapture Mesopotamia, perhaps during one of the Fitnas.
Even having lost Mesopotamia, the Caliphate still holds Egypt and Syria. No doubt the Persians would consider advancing into Syria... but then, the Byzantines also desired to reclaim Syria. Thus the threeway balance of power- if one of three holds all of Syria, it is has to defend on two fronts against the other two. I'm not sure how stable this dynamic would be, it certainly couldn't remain indefinitely. But if one of the three advances deep into another's territory, it leaves itself deeply exposed to the third- so it might endure for some time.
Aside from Syria, there are also fronts on which only two of the three would be at odds. The Caliphate and Byzantines would be vying over the Maghreb and Mediterranean islands, the Byzantines and Sassanids would likely to come into conflict over Eastern Anatolia/the Caucasus, and there would no doubt be Arab raids into Mesopotamia.
Also, depending on how long the Caliphate holds Mesopotamia before Persia reconquers it, there will be a substantial population of Muslims there. It's possible that the Persians would slaughter/forcefully convert them. But they might instead repeat their strategy with Christians(where they favoured Nestorian Christianity, so as to drive a wedge between Christians living in their territory and Roman Christians). Even IOTL early Islam was faced with schismatic sects like Shia and Kharijites, and conveniently they were most successful in areas that are easily accessible to Persia(Shia in Mesopotamia and the East coast of Arabia, and the Ibadis(Kharijite descendants) in Oman.
This would also have implications for the spread of Islam along Indian ocean trade routes- if the Persians choose to favour Shia and Kharijite factions in Eastern and Southern Arabia, then conversion to Islam due to the influence of Arab traders in Southern India and South East Asia may follow those denominations rather then being Sunni. Or perhaps the split would inhibit the spread of Islam along trade routes entirely?