Factbox: History of Iranian-Saudi relations

(Reuters) - The United States began the trial this week of two Iranians it said planned to kill the Saudi ambassador in Washington, raising the heat in an already tense relationship between Tehran and Riyadh.

Here are some details on the ups and downs of relations over the last 20 years:

* 1987 - MECCA

-- Relations between Saudi Arabia and Iran, severed by King Fahd in April 1988, were strained almost to breaking point in July 1987 when 402 pilgrims, 275 of whom were Iranian, died during clashes in the Muslim holy city of Mecca.

-- In the next year there were few signs of a cooling of tempers as Riyadh frequently warned Tehran it would not tolerate a repeat of violence in the Muslim world's holiest shrine.

-- Protesters took to the streets of Tehran, occupied the Saudi embassy and set fire to Kuwait's embassy. A Saudi diplomat, Mousa'ad al-Ghamdi, died in Tehran of wounds sustained when he fell out of an embassy window and Riyadh accused Tehran of delaying his transfer to a hospital in Saudi Arabia.

* 1997 - A SUMMIT

-- Then-Crown Prince Abdullah visited non-Arab Iran for an Islamic summit in December 1997, becoming the highest-ranking Saudi to do so since the 1979 Islamic revolution.

* 1999 - BETTER TIMES

-- King Fahd congratulated Iranian President Mohammad Khatami on his election victory in 2001, saying it was an endorsement of his reformist policy. Khatami, a Shi'ite Muslim cleric, worked for rapprochement with Saudi Arabia after winning his first landslide in 1997 and ending two decades of tense relations that followed Iran's 1979 Islamic revolution.

-- Khatami visited Saudi Arabia in 1999 on the first visit by an Iranian president since the revolution. The two countries sealed better relations with a security pact in April 2001.

* 2003 - REGIONAL FEARS

-- The invasion that toppled Saddam Hussein in Iraq empowered the country's Shi'ite majority and resulted in a shift in its political alignment toward Iran.

-- The war between Israel and Lebanese group Hezbollah, which receives funding from Iran, confirmed Saudi fears that Iran was creating a new network of alliances around the region that threatened Saudi and U.S. interests.

-- Iran's nuclear energy program deepened Saudi fears that Tehran under Khatami's successor President Mahmoud Ahmadinejad was bent on dominating the Gulf region and boosting its Shi'ite populations.

-- Saudi Arabia told an Iranian envoy in January 2007 that Iran was putting the Gulf in danger, in a reference to the Islamic Republic's conflict with the United States over Iraq and its nuclear program.

* 2011 - ARAB SPRING

-- Saudi Arabia watched in horror as pro-democracy uprisings moved westward from Tunisia and Egypt to the Gulf. Protests in Bahrain were seen as a red line because of fear that the island's Shi'ite majority would take power and ally the country with Iran, as happened with Iraq after the 2003 invasion.

-- Saudi Arabia sent troops to help put down the protests, which governments in both countries said helped save Bahrain's Arab identity.

-- U.S. diplomatic cables released by WikiLeaks showed Saudi leaders, including King Abdullah, pushing Washington to take a tough stance, including the possible use of military force.

-- Saudi Arabia accused some Shi'ites in its Eastern Province of cooperating with a foreign state -- meaning Iran -- to sow dissension, after clashes between police and Shi'ites.

-- Washington said it had uncovered an Iranian plot to assassinate the Saudi ambassador to the United States. Riyadh said the evidence was overwhelming and Tehran would pay a price.

(Writing by Andrew Hammond; Editing by Michael Roddy)