Final answer:
The U.S. president when the CIA backed the 1953 coup in Iran was Dwight D. Eisenhower. This intervention led to the installation of Shah Mohammad Reza Pahlavi, who ruled with an iron fist and U.S. support.
Step-by-step explanation:
The president of the United States when the CIA supported a coup d'etat against the democratically elected Prime Minister of Iran, Mohammad Mossadegh, in 1953 was Dwight D. Eisenhower. The operation, known as Operation Ajax, led to the overthrow of Mossadegh after he nationalized the oil industry, which was previously controlled by British companies. The CIA's involvement in the coup reinstated the monarchy under Shah Mohammad Reza Pahlavi, who then ruled Iran with an authoritarian regime supported by the U.S. government. This intervention played a significant role in the geopolitical dynamics of the region and the perception of the United States in Iran, eventually contributing to the Iranian Revolution of 1979.
In the context of international relations during the Cold War, the actions of Mossadegh were viewed with concern by the U.S. and the U.K as they feared Iran could align more closely with the Soviet Union. The nationalization of the oil industry posed a direct threat to Western economic interests and was used to justify the coup. Post-coup, the shah's aggressive policies along with a powerful secret police force, SAVAK, led to increasing dissatisfaction among Iranians, which culminated in the shah's overthrow in 1979.