The POD should happen after Persian conversion to Shia.
If an Empire that calls itself Persian (it could Be the Safavids, the Qajars, or the Pahlavids) could conquer and hold, in addition to the OTL territory of Iran, Tajikistan, part of Afghanistan, the Caucasus, part of Iraq, Bahrain, etc., how much of these territory could Iran keep in the dawn of Nationalism?
It is mainly about the ability to assimilate and creating identities. Would people with more cultural, religious and lingual similarities with the Iranains be assimilated easier than the rest of the populace?