I've heard that Iran was mostly Sunni before the Safavids came to power, and that they converted the country to Shi'ism with quite heavy handed tactics. A lot of people say that it was a forcible conversion that killed a lot of people and made the Safavids a lot of enemies in and out of Iran.
Maybe I've been hearing from biased sources, but it does prompt me to ask, why did they want to change the religion of the country, especially if it took a lot of violence and government repression? Was there a high level political purpose to it?
I just briefly addressed this question in a related post, here—but essentially, no, the switch from Sunnism to Shiʿism (though these categories were much less distinct before the sixteenth century) was not done for political purposes. Rather, it was because the Safavids and their military supporters had been ideologically committed to an extreme form of Shiʿism prior to their conquest of Iran as part of the Sufi order which the Safavids headed before their rise to political prominence.