Because Iran’s prime minister, Mohammad Mosaddegh, nationalized the country’s oil industry, challenging British economic interests.
America, for its part, was increasingly concerned about communist influence during the Cold War, not oil. In fact, its ventures with Saudi Arabia was the cause of the nationalization by Iran’s prime minister, because they felt being taken advantage by Britain.
Are you saying Britain is hence ultimately responsible?
According to Claude:
Significant responsibility
• 1914, Nigeria: Britain merged two protectorates but governed them separately → the Biafran War (1967–70).
• 1915–22, Iraq/Middle East: Britain made contradictory promises and drew borders with France → coups, the 1958 revolution, the 1990 invasion of Kuwait.
• 1917–48, Palestine: Britain made irreconcilable pledges and left without a settlement → the Arab–Israeli conflict.
• 1920–21, Ireland: Partition was drawn to give unionists a built-in majority → the Civil War and the Troubles.
• 1930–56, Sudan: Britain sealed off the south, then rejoined it to the north at independence → two civil wars and South Sudan's secession.
• 1947, India/Pakistan: A rushed exit and hastily drawn borders → partition violence, the Kashmir dispute, and Pakistan's 1971 breakup.
Partial responsibility
• 1935–48, Burma: Britain ruled the hill peoples separately → decades of insurgency, though the Japanese occupation and military rule matter as much.
• 1953, Iran: Britain co-led the coup with the CIA → the Shah's autocracy and the 1979 revolution, with the US sharing the lead.
• 1954–63, Somalia: Britain handed Somali-inhabited lands to Ethiopia and Kenya → border wars, though Italy, the Cold War and the dictatorship weigh more.
• 1960, Cyprus: Divide-and-rule tactics and an unworkable constitution → the 1974 division, with Greece and Turkey as central actors.
• 1965–79, Zimbabwe: Britain didn't confront the Rhodesian breakaway and deferred land reform → the later collapse, which was mostly the ruling party's doing.
• Also arguable: Sri Lanka, Fiji, Aden/South Yemen, and the Durand and McMahon border lines.