Saudi Arabia’s decision Sunday to sever diplomatic relations with Iran reflects long-running suspicions and outright hostility between the Middle Eastern rivals. The incident began when Saudi Arabia announced a day earlier that it had executed a prominent Shiite cleric, and that led an angry mob to ransack the Saudi Embassy in Tehran, the Iranian capital. But tensions between the two powers stretch back decades and are rooted in intense ideological and strategic competition. Saudi Arabia is Sunni. Iran is Shiite. And both are regional heavyweights whose destructive rivalry is fueling civil wars in Syria and Yemen and feeding tensions between Sunni and Shiite Muslims the world over. Below are some important moments in the history of Saudi-Iranian relations.

1) 1980-1988 Iran-Iraq War

Saudi Arabia was never a big fan of Saddam Hussein, but it gave financial and political backing to the Iraqi dictator’s destructive war with Iran that began in 1980 and resulted in more than a million deaths. Leaders in Riyadh feared that the Shiite theocracy that rose to power in Iran in the 1979 revolution would attempt to spread its fiery brand of religious politics across the region.

2) 1987 Hajj Clashes

Saudi Arabia severed relations with Iran in 1988 following tensions that came to a head after protesters in Tehran stormed the Saudi Embassy, an incident that left one Saudi diplomat dead. That crisis started with clashes between security personnel and Shiite pilgrims in the Saudi city of Mecca during the 1987 hajj pilgrimage in which 275 Iranians died.

3) 1996 Khobar Tower Bombings

A truck bomb that killed 19 U.S. servicemen in the Saudi city of Khobar was later linked by a three-year FBI investigation to Iranians and members of Lebanon’s Iran-backed Hezbollah militia. In 2006, a U.S. judge ruled that Iran and Hezbollah were responsible for the bombing.

4) 2003 U.S. Invasion of Iraq

Saudi leaders were livid over U.S. President George W. Bush’s decision to invade Iraq – a move that empowered Iraq’s Shiite-majority population and enabled Iran to dramatically expand its political, military and cultural influence in the country. Although Saddam Hussein invaded Kuwait in 1990 and threatened something similar against Saudi Arabia, officials in Riyadh saw his Iraq as better than what the country would eventually become: a firm Iranian ally.

Saudi Arabia says it has executed a prominent Shi'ite Muslim cleric and dozens of al Qaeda members. (Reuters)

5) 2011 Bahrain Uprising

Bahrain’s majority Shiite population led an Arab Spring uprising calling for more political rights that was violently put down by the tiny Arab Gulf country’s Sunni monarchy and an allied military force led by Saudi Arabia. That force, which included Saudi and UAE troops, sought to prevent what officials in Riyadh feared was an Iranian attempt to exert influence over Bahrain through its Shiites.

6) 2011-Present Syrian Conflict

Saudi Arabia and Iran back opposing sides in the Syrian civil war – the most destructive conflict in the region, and one that has empowered radical groups such as the Islamic State and created a huge refugee crisis that threatens to destabilize neighboring countries. Iranian soldiers and allied Hezbollah militants are fighting alongside President Bashar al-Assad forces against Syrian rebel groups, a number of which receive money and weapons from Saudi Arabia.

7) 2015 Hajj Stampede

A stampede during the hajj pilgrimage in Saudi Arabia killed as many as 2,411 people, including 450 Iranians. It took weeks to identify bodies, and officials in Iran accused their Saudi counterparts of using the disaster to kidnap Iranians who went missing during the chaos.

Saudi Arabia and Iran have held long-running suspicions and outright hostility towards each other. Here's why. (The Washington Post)