FACTBOX: Ups and downs in U.S.-Iranian relations

(Reuters) - Iran's chief nuclear negotiator said he hoped for "good and constructive" talks when he left Tehran on Friday to meet world powers in Geneva on the country's disputed nuclear program.

The United States is sending an envoy to the talks for the first time, seeking to underline to the Islamic Republic and others Washington's stated position that it wants a diplomatic solution to the impasse.

Here are some details of 55 years of turbulent relations between Iran and the United States.

1953 - A COUP

-- In August 1953, the CIA helped orchestrate the overthrow of Iran's popular Prime Minister Mohammed Mossadegh, restoring the Shah, Mohammed Reza Pahlavi, to power.

-- Washington acted after Britain, opposed to Mossadegh's policy of nationalizing the British-controlled oil industry, convinced U.S. officials the prime minister was turning to communism. As Britain's power faded, the United States became the symbol of what many Iranians saw as Western imperialism.

1972 - A VISIT

-- A 1972 visit by U.S. President Richard Nixon cemented a close strategic relationship between Iran and the United States. Iran spent millions of petro-dollars buying some of the most advanced U.S. military equipment. But opposition to the Shah, led by exiled cleric Ayatollah Ruhollah Khomeini, mounted.

1979 - A REVOLUTION

-- After months of increasingly bloody clashes between protesters and troops, the Shah fled into exile in January 1979. The next month, Khomeini returned to Iran in triumph to seal victory for a revolution whose mantra was "Death to America".

-- In November 1979, Iranian students seized the U.S. embassy in Tehran and took 90 hostages; 52 were held captive for 444 days, prompting Washington to break relations in 1980.

1986 - AN ARMS DEAL

-- U.S. President Ronald Reagan admitted to secret arms deals with Iran that broke a U.S. embargo. The trade was aimed at winning the release of Americans held by pro-Iranian Shi'ite militants in Lebanon. Money from the sales was secretly passed to U.S.-backed Contra guerrillas in Nicaragua. At the time, Iran was embroiled in its 1980-88 war with former President Saddam Hussein's Iraq, in which the United States gave increasing support to Baghdad.

1997 - A MODERATE

-- Iranian voters swept reform-minded President Mohammad Khatami to power. He promoted a "dialogue among civilizations". During his term, Iranians staged an impromptu vigil in Tehran when hijacked planes struck U.S. targets on September 11, 2001.

-- After those al Qaeda attacks, Iran offered support in a U.S.-led war to topple Afghanistan's Taliban leaders shielding al Qaeda chief Osama bin Laden. Iran helped ensure the success of a multilateral postwar conference on Afghanistan's future. In January 2002, U.S. President George W. Bush branded Iran part of an "axis of evil" and accused it of seeking nuclear weapons.