WASHINGTON - President George W. Bush on Wednesday said the United States would play a leading role in resolving the international impasse with Iran over its nuclear program.

Following is a timeline of key events in the troubled US-Iranian relationship:

- 1953: Iran's nationalist prime minister Mohammed Mossadeq is overthrown in a coup engineered by US and British intelligence services aimed at preventing nationalization of the country's lucrative oil industry.

Western powers subsequently back the Iranian monarch, Shah Reza Pahlavi, who sets up a widely feared secret police organization.

- 1979: After two years of rising unrest culminating in a mass uprising, the shah is forced to flee the country.

The Shiite Muslim cleric Ayatollah Ruhollah Khomeini, who played a key role in the revolution from exile in France, returns to Iran and sets up an Islamic republic.

In November, militant students enter the US embassy in Tehran and take all 63 staff members hostage. Some are later released.

- April 1980: The United States severs diplomatic relations with Iran. They have remained broken ever since.

US president Jimmy Carter orders a secret airborne operation to try to free the hostages. It fails disastrously, with the deaths of eight US troops.

- September 1980: The forces of then-Iraqi president Saddam Hussein attempt to profit from Iran's revolutionary turmoil by making a land grab in Iran's oil-rich southwest. The conflict rapidly escalates into full-scale war, with Western powers and many Arab governments secretly supporting Iraq against Iran.

- January 1981: The 52 remaining US hostages are freed from the Tehran embassy after negotiations, ending 444 days in captivity.

- 1987-88: Iranian attempts to hit tankers in the Gulf as part of the war with Iraq spark military clashes with US forces in the region.

In July 1988, a US warship shoots down an Iranian passenger plane which it had allegedly mistaken for a warplane. All 290 people on board are killed.

In the same month, Iran accepts a UN resolution calling for a ceasefire. The eight-year war has left at least one million dead.

- August 1990: Saddam's forces invade Kuwait, setting off a series of events which will bring about two US-led military interventions in Iraq, culminating in the current conflict.

- January 2002: US President George W. Bush identifies Iran, along with North Korea and Saddam's Iraq, as part of what he calls an "axis of evil."

- December 2002: Even as it is preparing to invade Iraq, the US administration says Iran may be developing nuclear weapons.

- June 2005: Disputed elections bring Mahmoud Ahmadinejad, an anti-Western hardliner, to power. Iran's hand is strengthened by US woes in neighboring Iraq and a tight world oil market.

- August 2005: US President George W. Bush makes the first of several statements in which he refuses to rule out using force against Iran, where Ahmadinejad formally takes office.

- January 2006: Iran announces it is resuming uranium enrichment activities, despite an agreement in principle to voluntarily suspend them.

- April 2006: Iran holds military maneuvers in the Gulf. Unconfirmed US press reports say the Bush administration could use tactical nuclear weapons against Iran.

- May 2006: Iran threatens to pull out of the Nuclear Non-Proliferation Treaty, while the US and its allies seek UN Security Council action to force Tehran to give up its nuclear program.

Ahmadinejad writes to Bush to "propose new ways" to resolve tensions between the arch foes, but Bush says the Iranian leader's letter fails to respond to international concern about Tehran's nuclear program.

UN Secretary General Kofi Annan says the United States must talk directly to Iran about its nuclear program, as Tehran will not negotiate seriously if Washington is not involved.

Bush says the United States will play a leading role in resolving the international impasse with Iran over its nuclear program.

05/31/2006 17:20 GMT

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