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Athens and Sparta.

Greece and Persia.

England and France.

And not long ago, the US and the Soviet Union. The list goes on.

In the second decade of the third millennium, the world stage looks set for a new epic animus, this time between the Kingdom of Saudi Arabia and the Islamic Republic of Iran, as Saudi severed diplomatic ties with Iran this week.

Religion, history, geopolitics... and oil

Relations between Iran and Saudi Arabia have been strained over the years due to different interpretations of Islam, aspirations for leadership in the Islamic world, oil export policy, geopolitics and relations with the US, Israel and the West in general.

Both Saudi Arabia and Iran are Muslim-majority nations and rule through Islamic law, but their relations are fraught with hostility, tension and confrontation, due to differences in agendas strengthened by their differences in faith.

Saudi Arabia is a conservative Wahhabi Sunni Islamic kingdom with a tradition of close ties with the United States and the United Kingdom.

Iran is a Shia Islamic Republic founded in an anti-Western revolution in 1979 against a pro-Western regime. Both Saudi Arabia and Iran have aspirations for leadership of the Islamic world, but have different visions of stability and regional order.

After the Islamic Revolution, relations deteriorated considerably after Iran accused Saudi Arabia of being an agent of the US in the Gulf region. Even the use of the term "Gulf" is loaded – use "Arabian Gulf" and prepare for accusations of pro-Saudi bias; use "Persian Gulf" and be accused of supporting Tehran's regional agenda.

Meanwhile, Saudi Arabia is concerned by what it sees as Iran's consistent desire to export its revolution to expand its influence within the region and beyond.

Both countries are also major oil and gas exporters and have clashed over energy policy.

Saudi Arabia, with its large oil reserves and small population, has a greater interest in taking a long-term view of the global oil market and incentives to moderate prices.

By contrast, Iran often focuses on high prices in the short term.

Timeline of tension

Pre-1979 revolution: Under the rule of Shah Mohammad Reza Pahlavi, Iran had rocky relations with Saudi Arabia, though they improved towards the end of his reign. Both were original members of the oil cartel OPEC.

Post-revolution: After the overthrow of the shah and takeover of the US embassy in Tehran, Saudi Arabia quickly became Washington's top ally in the region, after Israel. In the ensuing 1980s war between Iran and Iraq, Saudi Arabia backed Iraq – despite concerns about dictator Saddam Hussein.

1987 Hajj riots: The annual pilgrimage to Islamic holy sites in Saudi Arabia saw bloodshed when Iranians held a political demonstration. Iranian pilgrims later battled Saudi riot police in violence that killed at least 402 people. In Tehran, mobs attacked the Saudi, Kuwaiti, French and Iraqi embassies, ransacking the first two.

In 1988, Saudi Arabia severed ties with Iran, citing the 1987 hajj rioting and Iran's attacks on shipping in the Gulf. Iranians responded by boycotting the hajj in 1988 and 1989.

The two countries restored diplomatic ties in 1991.

2003 invasion of Iraq: The invasion that toppled Saddam Hussein in Iraq empowered the country's Shia majority and resulted in a shift in its political alignment towards Iran.

2011 post-Arab Spring: Saudi Arabia sent troops to help Bahrain quash mass pro-democracy protests, fearing the mostly Shia opposition would align with Iran. The two countries later accused Tehran of fomenting violence against Bahraini police.

Saudi Arabia accused some Shia Muslims in its Eastern Province, including Sheikh Nimr, of cooperating with a foreign state – meaning Iran – to sow dissent, after clashes between police and locals.

That year as well, 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.

Later, worries about Iran resumed in Saudi Arabia amid international sanctions against Tehran over its contested nuclear programme and the increasingly harsh rhetoric of Iranian President Mahmoud Ahmadinejad.

Post-2012 proxy wars: Saudi Arabia became the main supporter of rebels fighting to topple Iran's ally, President Bashar al-Assad, in Syria. Riyadh accused Assad of "genocide" and Iran of being an "occupying power". Tehran accused Riyadh of backing "terrorism".

March 2015: Saudi Arabia began a military campaign in Yemen to stop the Houthis, allied to Iran, from taking power. Riyadh accused Iran of using the militia to stage a coup d'etat. Tehran said Riyadh's airstrikes targeted civilians.

2015, hajj disaster: On September 24, a stampede and crush struck the annual hajj pilgrimage in Saudi Arabia. While the kingdom says 769 pilgrims were killed, an Associated Press count showed more than 2,400 people died. Iran says at least 464 of its pilgrims were killed and blamed Saudi Arabia's "incompetence" for the deaths.

2016, Nimr's execution: On January 2, Saudi Arabia executed Shia cleric Sheikh Nimr al-Nimr and 46 others – the largest execution carried out by the kingdom in three and a half decades.

The execution of Nimr sparked protests across the Middle East and attacks on Saudi diplomatic facilities in Iran.

Saudi Arabia and its close allies responded by announcing it was severing diplomatic ties with Iran over the attacks. Business links and travel will also be cut between the two nations, though Saudi officials have said Iranians would still be welcome at the annual hajj pilgrimage.

Agencies contributed to this report