Timeline of how we got from dynastic monarchy to the Islamic Republic of Iran:
1891: tobacco fatwa
1904-1910: clerics heavily supported a constitutional revolution. It was semi-successful but things never really stabilized and there was a lot of power struggle between the new parliament and the Qajari Shah.
1925: Reza khan Pahlavi, a colonial in the army, launched a successful coup and became Reza Shah.
1939: the US forced him to abdicate his throne to his son and flee the country. He had a nasty habit of getting friendly with some dude named Adolph. The name Iran is literally the Persian derivative of Aryan. Ethnic Persians are technically Caucasian, not Arab. His son was largely an absent ruler, preferring to spend his time in Switzerland and France. The parliament regained proper democratic power, the country started to flourish, things were getting better.
1953: the goddamn Brit’s convinced us that the Prime Minister of Iran, Mohammad mossadegh, was leaning commie. The still fled fleeing CIA took the opportunity to see how effective it could be. Operationally, it was a semi-success. The first attempt to oust mossadegh failed but the second attempt, a few days later succeeded. The operation only cost the modern equivalent of a few hundred grand. Fun facts: the cia officer who directed the operation was named Kermit Roosevelt.
less fun fact: a few months before the coup, GHW Bush formed his first oil company with the help of a CIA officer.
1953-1979: Mohammad Reza Pahlavi, now fully in control of the country and militarily funded by the west did fucked up shit like having the military shoot protestors for not wearing ‘western’ hats. He forced westernism in the country by neutering religious authority as much as possible. Khomeini was repeatedly arrested and ultimately banished from the country. Economically, Iran wasn’t doing well because Mo liked to spend everything he had on himself and the military. The common folk, whether Shia, Sunni, Christian, Jew, or secular were not doing well. Khomeini saw this and formed alliances throughout religious communities and business organizations that ultimately lead to the success of the Islamic revolution.
Despite mainstream reporting, the Saudi-Iran tension has little to do with the Sunni-Shia schism. Saudi is fucking terrified of any Islamic government that isn’t a monarchy. If a semi-democratic Islamic government can be successful outside Saudi, it could stir the fires of revolution in Saudi. The same reason they oppose the Egyptian Muslim brotherhood.