Iran and Azerbaijan are the dominant Shiite majority states in the world today, and substantial parts of the populations of modern-day Iraq and Bahrain practice Twelver Shiism too. However, from what I understand, Shiites only became so prevalent in this region as a result of the conversion of the Safavid leadership to Twelver Shiism from Sunni Islam and the dynasty's sibsequent efforts to propagate their embraced faith.
The single biggest consequence of the Safavid decision to embrace and propagate Shiism seems to me to have been to convert most Iranians to Shiism. But, I ask all of you, what if the Safavids had never embraced Shiism?