In 1953, the CIA undertook a coup to try and overthrow
the Iranian democracy and install the Shah, who subsequently became a tyrannical ruler of Iran. This top secret mission was called Operation Ajax. Support of this coup is why the Iranian public hated the U.S. This was the first seed of the conflict between the two countries.
On November 4, 1979, the American Embassy in Tehran was taken by a radical religious group of Islamic fundamentalist students who were supported and led by the Ayatollah Khomeini. They were trying to protest the American government admitting the Shah into the U.S. for medical treatment. Huge mobs of people chanted anti-American slogans outside the embassy gates. Students started climbing over the wall, and one cut the lock on the gate. The American Marines were under orders not to shoot, so they retreated into the nearest building. With 500 people now in the embassy, the diplomats locked themselves in the vault and started to shred classified documents.
The diplomats in the passport issuing station, which was walled off from the rest of the complex, were alerted and got out to the open street. They split into two groups of five and six. The group of six was captured by militants and was brought back to the embassy. The group of five got to a American owned apartment and eventually escaped to freedom. After the first several weeks, 13 people were set free leaving 51 men and 2 women remaining as hostages, thus starting the long 444 days.