In the Iranian Revolution of 1979, Islamic fundamentalists and others rose up and overthrew the government of the Shah of Iran. The Shah left the country never to return and the country became a theocracy led by Muslim religious leaders.
Iran had been a monarchy since 1953. In that year, the United States had helped to foment a coup against the government of Iran because it was seen as sympathetic to communism. The US installed the Shah as the monarch. The Shah came to be widely disliked within Iran because his government was somewhat repressive. Importantly, his government was seen as anti-Muslim and it tended to be repressive towards clergy that led protests against the government. One of these clerics, the Ayatollah Khomeini, was forced into exile.
In 1978, massive protests against the Shah’s rule broke out and could not be completely suppressed. The Shah eventually left the country for medical treatment in the US. Khomeini returned from exile. Khomeini’s faction eventually overcame both secular groups and more moderate religious groups. This faction came to lead the country, which became a theocracy.
This change from a secular and authoritarian government to a theocracy was the main action of the Iranian Revolution.