Forty years ago, Ayatollah Ruhollah Khomeini spearheaded an Islamic Revolution that deposed Iran's shah. The country's authoritarian monarchy was replaced by a repressive theocracy, and almost overnight, Iran was transformed from a crucial U.S. ally to bitter foe.
In Pictures: Iran's 1979 Islamic Revolution

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Demonstrators hold up a poster of exiled Islamic leader Ayatollah Ruhollah Khomeini as protests in Tehran grow larger. This anti-Shah demonstration on December 10, 1978 took place near the Shayah monument which was built to commemorate the monarch's rule and power.

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A mullah and others in Tehran's bazaar on December 16, 1978, reading the latest words from exiled Islamic leader Ayatollah Ruhollah Khomeini.

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An overturned truck with a Pepsi logo burns in the center of Tehran during riots which paralyzed the city on December 27, 1978.

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Anti-Shah protests continue to grow as demonstrators hold a poster of Ayatollah Ruhollah Khomeini during mass protests on January 1, 1979. Khomeini had spent 14 years in exile. Cassette tapes of speeches by the 76-year-old Khomeini were smuggled into the country, fueling the unrest.