http://en.wikipedia.org/wiki/BPBut in March 1951, the pro-western Prime Minister Ali Razmara was assassinated. The Majlis of Iran (parliament) elected a nationalist, Mohammed Mossadeq, as prime minister (you remember him...he was man of the year in 1951
http://en.wikipedia.org/wiki/Mohammed_Mossadeq) In April, the Majlis nationalized the oil industry by unanimous vote. The British government contested the nationalization at the International Court of Justice at The Hague, but its complaint was dismissed.
The British government decided that the only way to regain its control of Iranian oil (which it regarded as a vital national interest), was to remove Mossadeq from office. It aimed to replace him with a more friendly regime led by the Shah, Mohammed Reza Pahlavi. Its problem was that it lacked the means to do so without American support. But it was clear the U.S. government would never support a coup d'état designed only to protect Britain's commercial interests. So the British played on America's then paranoia about the Communist threat by producing bogus evidence that Mossadeq was scheming to bring Iran into the Soviet sphere of influence. In early 1953, incoming U.S. President Dwight D. Eisenhower authorized the Central Intelligence Agency (CIA) to overthrow the Iranian government. The CIA conspiracy, involving the Shah and the Iranian military, became known by its codename, Operation Ajax.
On August 19, 1953, Mossadeq was forced from office by a CIA-orchestrated military coup. He was replaced by pro-Western general Fazlollah Zahedi. The Shah, who had left the country briefly to await the outcome of the coup, returned to Iran. He abolished the democratic Constitution and assumed autocratic powers. The AIOC became the British Petroleum Company in 1954, and resumed operations in Iran. However, as a part of the Anglo-American coup d'état deal, it was not allowed to monopolize Iranian oil as before. It was limited to a 40% share in a new international consortium. For the rest, 40% went to the five major American companies. 14% went to Royal Dutch Shell. 6% went to Compagnie Française des Pétroles.