On this weblog, there is sometimes a silly debate between Hindu nationalists and anti-Hindu anti-nationalists about the scope and size of Indian and Hindu cultural spheres (the terms “Indian” and “Hindu” being interchangeable for much of history). I stumbled onto this comment from Isidorus of Charax (a Greek subject of the Parthian Empire) in his Parthian Stations:
19. Beyond is Arachosia, 36 schoeni. And the Parthians call this White India; there are the city of Biyt and the city of Pharsana and the city of Chorochoad and the city of Demetrias; then Alexandropolis, the metropolis of Arachosia; it is Greek, and by it flows the river Arachotus. As far as this place the land is under the rule of the Parthians.
Here Isidorus is alluding to the Indo-Greeks who were dominant in much of Afghanistan and Bactria to the north. “White India” in the first few centuries A.D. seems to have meant the Helmand Valley, modern southern Afghanistan.
Apparently in the Avesta this region is asserted to be staunchly Zoroastrian. Either there is confusion and misrepresentation, or, Zoroastrianism retreated during this period. Though this specific case can be dismissed, it does seem to be a fact that under the Parthians and Sassanids the eastern and northeastern fringe of the Iranian world was coming under strong Buddhist influences. In the early period, Buddhism was, of course, the “Indian religion” (just as Islam was and is the “Arab religion”), so presumably, it was a vector for Indian cultural influence.