In 602AD, a mere 40,000 arabs conquered Iran who had an army of 1,000,000 (embroiled in a civil war tho), and forced them to convert by sword. Mind you, Iran at the time included all lands ending with "stan" back then, from Afganistan to Uighurstan, and down around the Persian gulf to Yemen.
THIS is the cause of the Sunni/Shiite rift. The Ali vs Hussein bit was just an excuse. The two sects are ethno/cultural based, and Shiia is really Zoroastrian dressed up as islam,,,
As a result of the slaughter of those who refused to convert, only 150,000 Zoroastrians exist today, most in India where they had literally fled to for their lives.
Do you not know ANY of YOUR religion's history?