April 1, 1979 — Iran officially proclaimed an Islamic Republic

Although the revolution against him began in January 1978, the Shah did not flee Iran until January of 1979. Ayatollah Khomeini returned from exile shortly thereafter, and while revolutionary and loyalist forces fought, the military declared itself neutral and sat out the fight.

On March 30 and 31, 1979, a referendum was held, and the Iranian people voted overwhelmingly to become a theocratic state. On April 1, it was proclaimed that the nation would henceforth be called the Islamic Republic of Iran, and Khomeini would be its president.

Imam Khomeini in Mehrabad.jpg
By Also uploaded by en:User:Sa.vakilian in en.wiki and by sajed.ir. It has released under GFDL license.
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As mentioned in:

We Didn’t Start The Fire — Billy Joel