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They are the two main branches of Islam — like Catholic and Protestant within Christianity. Both follow the Quran and Muhammad as the final prophet. The difference is not about theology so much as it is about authority: who has the right to lead the Muslim community, and what qualifies someone for that role.
CORE FAULT LINE: Iran is the world's only Shia-majority great power and sees itself as the protector of Shia communities everywhere. Saudi Arabia leads the Sunni world. This rivalry drives proxy conflicts from Yemen to Lebanon to Iraq.