Sources

Belligerents

After the Second World War, the USSR briefly occupied Iran.
The US forced the Soviets to withdraw in 1946.
In the 1950’s, the British and United States MI6 and CIA assisted the Shah in a coup d’etat, codenamed Operation Ajax.
The United States continued to back the Shah for the next two decades.
After the Iranian Revolution in 1979, the American Embassy was briefly occupied with its staff held hostage.
The Carter administration allowed the Shah to come to the US for medical treatment, angering the new de facto government in Iran, which publicly declared that the US was planning on assisting the Shah in another coup.

On February 14th, 1979, the Organization of Iranian People’s Fedai Guerrillas stormed the US Embassy and took a marine hostage, though he was released 6 days later with US pressure.
In planning the second attempt, the students observed the procedures of the Marine Security Guards and drew from their experience during the prior revolution. They also enlisted the help of police officers in charge of guarding the embassy and of the Islamic Revolutionary Guard Corps.

At 6:30 AM on November 4th, 1979, protestors, with the organization of some ringleaders, managed to cut in and break into the US embassy, eventually taking it over.
Islamists in Iran rounded up most of the absent embassy workers and returned them to the embassy, with some workers managing to take refuge in the British, Swedish, or Canadian embassy.
- Six American diplomats were smuggled out of Iran on January 28th, 1980, in a joint Canadian-CIA operation known as the Canadian Caper
Initially, the demands from the students were for the return of Shah Mohammad Reza Pahlavi from the US (where he was being treated for cancer), and for the United States to apologize for meddling in Iran’s affairs in the 1953 overthrow of the Prime Minister, Mosaddegh.
Documents were taken from inside the embassy and selectively published to portray America as planning some sort of interference with internal Iranian affairs, although it seems unlikely this was ever being planned or considered.
Internal factions in Iran used the union around the hostage situation to strengthen their positions inside the country, eventually leading to the adoption of a theocratic constitution and the purging of more moderate elements after the end of the hostage crisis.
The United States claims many external players were implicated in the takeover, including the Soviets, Cuba, Libya, and East Germany.

Iran published collections of documents found in the embassy in a series of books, titled Documents from the U.S. Espionage Den

The hostage-takers released women and minorities first, after declaring solidarity with “oppressed minorities.”
Yasser Arafat traveled to Tehran and assisted with the release of 13 other hostages.
The hostages endured general mistreatment as prisoners.