Saudi Arabia's execution on Saturday of Sheikh Nimr al-Nimr, an outspoken Shi'ite cleric who called for the overthrow of the Saudi royal family, triggered international condemnation and set off protests throughout the Middle East.
Hours after the execution, protesters stormed the Saudi Embassy in Teheran and set the building on fire. Iran's supreme leader, Ayatollah Ali Khamenei, ratcheted up the rhetoric, declaring: "God's hand of retaliation will grip the neck of Saudi leaders."
By Sunday, Saudi Arabia cut all diplomatic relations with Iran, and gave Iranian diplomats 48 hours to leave the kingdom.
The execution of the cleric quickly escalated into a diplomatic crisis between Iran and Saudi Arabia because the two regional rivals have been fighting a cold war for over a decade.
While the conflict is partly rooted in the Sunni-Shi'ite schism within Islam, it is mainly a struggle for political dominance of the Middle East between Shi'ite-led Iran and Sunni-led Saudi Arabia.
This series of proxy battles - in Iraq, Syria, Yemen, Lebanon and Bahrain - have shaped the Middle East since the United States invaded Iraq in 2003. In Iraq, neighbouring Sunni regimes backed Sunni militants, while Iran supported the new government and Shi'ite militias. Saudi Arabia, which viewed Iraq as a bulwark against Iranian influence, tried to destabilise the Shi'ite-led government in Baghdad.
The House of Saud rests its legitimacy - and its claim of leadership over the wider Muslim world - on the fact that the kingdom is the home of Islam's two holiest cities, Mecca and Medina, where the religion was founded.
Like his predecessors, the newly installed King Salman has taken the title of "Custodian of the Two Holy Mosques", a reminder of his rule over Islam's most sacred shrines.
The Sauds, and the Wahhabi clerics who support them, construe political legitimacy on the mediaeval Islamic concept that Muslims owe obedience to their ruler, as long as he can properly apply Islamic law.
This view does not tolerate public dissent. The Sauds also want to associate Islam with its original Arab identity, even though Arabs have been a minority within the religion for centuries.
While the Saud dynasty views itself as the rightful leader of the Muslim world, Iran has challenged that leadership for decades and especially after the Islamic Revolution in 1979.
Although Saudi Arabia has a Sunni majority, its rulers fear Iran's potential influence over a sizeable and sometimes-restive Shi'ite minority, which is concentrated in the Eastern Province, where most of the kingdom's oil reserves lie. (Sheikh Nimr was a leader of the Shi'ite community, and, at times, he advocated secession for the oil-rich Eastern Province.)
In Bahrain, another American ally in the Gulf, the Shi'ite majority is chafing under Sunni rulers who also fear Iran's reach.
More broadly, since the US invasion of Iraq, the traditional centres of power in the Arab world - Egypt, Saudi Arabia and other Gulf states - have been nervous about the growing influence of Iran: its nuclear ambitions, its sway over the Iraqi government, its support for the militant groups Hizbollah and Hamas, and its alliance with Syria.
Hizbollah's strong performance against a far superior Israeli military during their summer 2006 war electrified the Arab world, and it offered a stark contrast to Arab rulers appeasing the US.
Since the 1930s, the House of Saud has managed a fraught pair of alliances: one with Wahhabi clerics who vilify America and the West, and whose ultra-conservative teachings are the official form of Sunni Islam in Saudi Arabia, and the other with the US, as a major global oil supplier and its most important ally in the Arab world.
Successive US administrations supported the Sauds and provided military assistance whenever aggressive neighbours like Iraq threatened the kingdom.
In 1990, when Saddam Hussein invaded neighbouring Kuwait, Washington sent half a million troops to Saudi Arabia and used it as a base from which to drive the Iraqis out of Kuwait.
The American military presence on Saudi soil enraged Islamic radicals, who decried the Sauds' decision to allow "infidel" Western forces into Islam's birthplace.
Osama bin Laden was among those who turned against the ruling family in 1990, and accused it of straying from Islam.
The conflict with Iran has only intensified since the Arab uprisings of 2011, when the House of Saud tried to choke off revolutionary momentum in the region.
After the wave of popular protests forced out dictators in Tunisia, Egypt and Libya, the House of Saud resorted to one of its time-honoured methods of shoring up internal support: generous handouts.
Then King Abdullah pledged to build a half-million housing units for low-income Saudis; awarded two extra months of salary to all government employees, who make up a majority of the national workforce; and created more public-sector jobs.
King Abdullah also granted about US$200 million to organisations controlled by the Wahhabi religious establishment, including the morality police. In turn, the kingdom's highest religious council issued a fatwa proclaiming that Islam forbids street protests.
The ruling family also played the Shi'ite card, declaring that the uprisings across the region were targeting Sunnis and being instigated by Iran.
The Saudi regime became more nervous when the revolutions spread to Yemen, on its southern border, and Bahrain, a Shi'ite-majority country ruled by a Sunni regime only 26km from the Eastern Province.
The Sauds accused Iran of supporting the Bahrain uprising, and sent troops in March 2011 across the causeway to help crush the pro-democracy movement.
Both Iran and Saudi Arabia increasingly see their rivalry as a winner-takes-all conflict. And as long as this rivalry endures, the scourge of sectarian warfare will not end.
Mohamad Bazzi is a journalism professor at New York University and a former Middle East bureau chief at Newsday, a New York newspaper
This article was first published on January 5, 2016.
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