As Kinzer puts it,

Historic as Mossadegh’s rise to power was for Iranians, it was at least as stunning for the British. They were used to manipulating Iranian prime ministers like chess pieces, and now, suddenly, they faced one who seemed to hate them....

[U.S. presidential envoy Averell] Harriman paid a call on the Shah before leaving Tehran, and during their meeting he made a discreet suggestion. Since Mossadegh was making it impossible to resolve the [Anglo-American Oil Company] crisis on a basis acceptable to the West, he said, Mossadegh might have to be removed. Harriman knew the Shah had no way of removing Mossadegh at that moment. By bringing up the subject, however, he foreshadowed American involvement in the coup two years later.

The 1953 CIA coup in Iran was named “Operation Ajax” and was engineered by a CIA agent named Kermit Roosevelt, the grandson of President Theodore Roosevelt. Capitalizing on the oil-nationalization showdown between Iran and Great Britain, which had thrown Iran into chaos and crisis, Kermit Roosevelt skillfully used a combination of bribery of Iranian military officials and CIA-engendered street protests to pull off the coup.

The first stage of the coup, however, was unsuccessful, and the shah, who had partnered with the CIA to oust Mossadegh from office, fled Tehran in fear of his life. However, in the second stage of the coup a few days later, the CIA achieved its goal, enabling the shah to return to Iran in triumph ... and with a subsequent 25-year, U.S.-supported dictatorship, which included one of the world’s most terrifying and torturous secret police, the Savak.

For years, the U.S. government, including the CIA, kept what it had done in Iran secret from the American people and the world, although the Iranian people long suspected CIA involvement. U.S. officials, not surprisingly, considered the operation one of their greatest foreign-policy successes ... until, that is, the enormous convulsion that rocked Iranian society with the violent ouster of the shah and the installation of a virulently anti-American Islamic regime in 1979.

It is impossible to overstate the magnitude of anger and hatred that the Iranian people had for the U.S. government in 1979, not only because their world-famous democratically elected prime minister had been ousted by the CIA but also for having had to live for the following 25 years under a brutal and torturous dictatorship, a U.S.-government-supported dictatorship that also offended many Iranians with its policies of Westernization. In fact, the reason that the Iranian students took control of the U.S. embassy after the violent ouster of the shah in 1979 was their genuine fear that the U.S. government would repeat what it had done in 1953.