What event caused the relationship between the United States and Iran to become hostile?

B - The 1979 Islamic Revolution caused the relationship between the United States and Iran to become hostile. The revolution led to the overthrow of the Shah of Iran, who was a U.S. ally, and the establishment of an Islamic Republic under Ayatollah Khomeini. This shift drastically altered U.S.-Iran relations, leading to events such as the Iran Hostage Crisis, where American diplomats were held hostage, further straining the two countries' relationship.