Demonstrations against the Shah commenced in October 1977, developing into a campaign of civil resistance that was partly secular and partly religious, and intensified in January 1978. Between August and December 1978 strikes and demonstrations paralyzed the country.
The Shah left Iran for exile in mid-January 1979, and in the resulting power vacuum two weeks later Ayatollah Khomeini returned to Tehran to a greeting by several million Iranians. The royal regime collapsed shortly after on February 11 when guerrillas and rebel troops overwhelmed troops loyal to the Shah in armed street fighting.
Iran voted by national referendum to become an Islamic Republic on April 1, 1979, and to approve a new theocratic constitution whereby Khomeini became Supreme Leader of the country, in December 1979.
The revolution was unusual for the surprise it created throughout the world: it lacked many of the customary causes of revolution (defeat at war, a financial crisis, peasant rebellion, or disgruntled military); produced profound change at great speed; was massively popular; and replaced a modernising monarchy with a theocracy based on Guardianship of the Islamic Jurists (or velayat-e faqih). Its outcome—an Islamic Republic "under the guidance of an extraordinary religious scholar from Qom"—was, as one scholar put it, "clearly an occurrence that had to be explained".
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