my review of a history of iran is up. one thing that surprises many people is that iran/persia as a shia domain is a function of the forced conversion of the sunni populace by the safavid turkish dynasty from eastern anatolia during the 16th century. though there were shia dynasties and regions within iran,

1) they were not notable in persistence vis-a-vis other regions
2) they tend to be temporally placed further back in time when shia-sunni distinctions were more fluid (e.g., the buyids were shia persians, but during a time before sunni islam had crystallized as a coherent counter-narrative to shi’ism).

p.s. two of the four schools of sunni jurisprudence were founded by ethnic persians, the hanafi (the most numerous) and hanbali (the favorite of salafists)