The C.I.A. has admitted orchestrating the 1953 coup that toppled Prime Minister Mohammad Mossadegh of Iran after he tried to nationalize his country’s oil wealth, which was controlled by Britain, according to a declassified document. The agency’s role in the coup has long been known, but George Washington University’s National Security Archive, which obtained the document under the Freedom of Information Act, said it was the most explicit C.I.A. admission yet. In 2000, Secretary of State Madeleine K. Albright admitted that the United States had “played a significant role.” President Obama made a similar admission in 2009.