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

(Reuters) - Envoys from the United States and Iran began a second round of talks in Baghdad on Tuesday on Iraq's security crisis, following up on a meeting in May between the two rivals that ended a long diplomatic freeze.

Following are key events in U.S.-Iranian relations in the past half century:

1953 - A COUP

In August 1953, the CIA helps orchestrate the overthrow of Iran's popular Prime Minister Mohammed Mossadegh, restoring the Shah of Iran, Mohammed Reza Pahlavi, to power. Washington is prompted to act after Britain, opposed to Mossadegh's policy of nationalizing the British-controlled oil industry, convinces U.S. officials the prime minister is turning to Communism. For many Iranians, the United States becomes the rising symbol for what they see as Western imperialism, as Britain's power fades.

1972 - A VISIT

A high-profile visit by U.S. President Richard Nixon in May, 1972 cements what has become a close strategic relationship between Iran and the United States. At this time, Iran is spending hundreds of millions of dollars buying some of the most advanced U.S. military equipment. But opposition is mounting from an exiled cleric, Ayatollah Ruhollah Khomeini.

1979 - A REVOLUTION

After months of increasingly bloody clashes between protesters and troops, the shah boards a plane in January 1979 and leaves Iran for permanent exile. The next month, Khomeini returns triumphant and seals victory for a revolution for which a lasting mantra is "Death to America". In November 1979, Iranian students seize the U.S. embassy in Tehran and capture 90 hostages; 52 are held captive for 444 days, prompting Washington to break relations in 1980.

1986 - AN ARMS DEAL

U.S. President Ronald Reagan admits to a secret arms deal with Iran in breach of the U.S. arms embargo. Money from the sales was secretly passed to Contra guerrillas in Nicaragua. Iran at the time is fighting a war that began in 1980 with Iraq, which receives support from Washington. The war ends in 1988.

1997 - AN ELECTION

Reform-minded President Mohammad Khatami sweeps to power seeking a "dialogue among civilizations". During his two terms in office lasting until 2005, Iranians stage an impromptu vigil in Tehran when hijacked planes slam into the World Trade Center towers on September 11, 2001 and the government offers support in a U.S.-led war to overthrow the Taliban in Afghanistan. After multilateral talks on Afghanistan attended by Iranian and U.S. officials, U.S. President George W. Bush brands Iran part of an "axis of evil".

2007 - A NUCLEAR ROW

The United States leads efforts to toughen U.N. sanctions on Iran after Tehran ignores a February 21 U.N. deadline to halt sensitive nuclear work, which Washington says is aimed at building atomic bombs. Tehran insists its plans are peaceful. President Mahmoud Ahmadinejad, elected in 2005, regularly berates the West in his speeches.

MARCH 10 2007 - IRAQ

Iran attends talks on March 10 in Baghdad that gather Iraq's neighbors and other countries, including the United States. Officials discuss Iraq's worsening security crisis. Washington routinely accuses Tehran of backing Iraqi militants, a charge Iran dismisses saying the U.S.-led invasion in 2003 is to blame for spiraling violence.