You start with a pure popular democracy that the people love .. aand then you piss of the USofA and UK by making independant decisions as a nation:

    In 1951, Mohammad Mosaddegh was appointed as the Prime Minister of Pahlavi Iran. After the nationalization of Iran's oil industry, he became enormously popular.

    He was deposed in the 1953 Iranian coup d'état, an Anglo-American covert operation that marked the first time the United States had participated in an overthrow of a foreign government during the Cold War.

    . . .the U.S.- and UK-instigated, Iranian army-led overthrow of the democratically elected Prime Minister Mohammad Mosaddegh installed the monarchical rule of the Shah, Mohammad Reza Pahlavi

leading to ~ 25 years of a US puppet government

    After the coup, the Shah became increasingly autocratic and sultanistic, and Iran entered a decades-long phase of controversially close relations with the United States and some other foreign governments. While the Shah increasingly modernized Iran and claimed to retain it as a fully secular state, arbitrary arrests and torture by his secret police, the SAVAK, were used for crushing all forms of political opposition.

which made the religuous fundementalist opposition popular enough to stage a coup and install a "supreme leader".

https://en.wikipedia.org/wiki/Iran

https://en.wikipedia.org/wiki/1953_Iranian_coup_d%27%C3%A9ta...