In a significant escalation of tensions, Saudi Arabia cut ties with Iran and expelled its diplomats on Sunday (Jan 3), a day after its embassy in Teheran was attacked to protest Riyadh's execution of prominent Shi'ite cleric Sheikh Nimr al-Nimr.
The move marks the biggest crisis in relations between the two regional powers since the late 1980s, when the Sunni-led Saudi Arabia suspended ties with Shi'ite-ruled Iran after its embassy was attacked following the death of Iranian pilgrims during clashes in the holy city of Mecca.
Here's what you should know about the latest tensions and the divide between Sunni and Shi'ite Muslims:
Who is Sheikh Nimr al-Nimr?
Nimr, a 57-year-old Shi'ite cleric from Saudi Arabia's oil-rich Eastern Province, was a well-known figure at anti-government demonstrations. He had criticised Saudi rulers in some of his sermons for their treatment of the kingdom's Shi'ite minority. In 2009, he threatened to lead Saudi Arabia's Shi'ite Muslims to secession, provoking a government crackdown in the Shi'ite eastern heartland.
In his sermons, Nimr was critical of Sunni and Shi'ite autocratic rulers alike, though he reserved some of his most scathing attacks for the Saudi and Bahraini royal families.
In a meeting with US diplomats in 2008, he sought to distance himself from Teheran, according to a cable released by WikiLeaks. Iran, like other countries, acts out of self- interest, and Saudi Shi'ites should not expect Iranian support based on sectarian unity, he said. The report describes him as a "second-tier political player" in the Eastern Province.
Nimr was arrested in 2012, a year after popular uprisings swept parts of the Middle East, and sentenced to death in 2014.
Why does his execution hold for Saudi Arabia?
While Sunni-majority Saudi Arabia largely escaped the unrest that spread across the Arab world from 2011, the country's Shi'ites, who say they suffer discrimination, have occasionally protested and clashed with security forces. Most Saudi Shi'ites live near some of the world's largest oil fields in the eastern region, and according to the CIA World Factbook, make up between 10 and 15 per cent of Saudi Arabia's population.
The execution "institutionalises tension in Saudi Arabia by creating a symbol for Shi'ite grievances", said senior foreign policy fellow Ibrahim Fraihat from the Brookings Doha Centre. "Not many people in the past saw him as the representative of the Shi'ite community, but now he has become one of the symbols of the tension between Shi'ite and Sunnis."
In 2015, militants from the Islamic State in Iraq and Syria (ISIS) took advantage of Saudi sectarian fault-lines and struck Shi'ite mosques in the Eastern Province. Shi'ites are a majority in neighbouring Bahrain, a small island that is home to the US Fifth Fleet. Bahraini authorities regularly accuse Iran of supporting extremist Shi'ite groups, a charge which the Islamic Republic denies.
Why carry out the execution given regional tensions?
Given the complex dynamics in the region in Yemen, Syria and Iraq, Nimr's execution was an illustration of Saudi Arabia's "get tough" policy against Iran and internal dissent, said Dr Scott Lucas, an Iran analyst and professor of international politics at Birmingham University. "The Saudis deliberately crossed the line by executing him, and to add insult to injury, they used the rhetoric that lumps him in with Al-Qaeda terrorists," he said.
Nimr was one of 47 men executed on Saturday (Jan 2). Many of them were Sunnis convicted of terrorism-related offences, whom Saudi authorities described using terms that it normally reserves for militant groups like Al-Qaeda and ISIS.
How did Iran react?
Iranian protesters armed with rocks and firebombs massed outside the Saudi embassy in Teheran on Jan 2 and set parts of the building on fire. The following day, Iran's Supreme Leader Ayatollah Ali Khamenei said Saudi rulers will face "the divine hand of revenge" for their actions. Mr Khamenei, the country's highest authority and a regular critic of Saudi policies, stopped short of saying Iran would take action.
Iranian President Hassan Rouhani, meanwhile, suggested that he did not seek to escalate the confrontation. While he denounced the execution, Mr Rouhani condemned the attack on the Saudi embassy as unjustifiable. Dr Lucas said the incident might "spiral quickly into a domestic fight" in Iran between hardliners and more moderate factions close to Mr Rouhani.
What may happen next?
Iran can choose to increase economic pressure on Saudi Arabia, which is already running big current account deficits, by flooding the international market with cheap oil. After last July's nuclear deal with Teheran, many Western embargoes, including on oil exports, are due to be relaxed soon. Ever lower long-term prices could definitively scatter the Saudi house of cards, with unforeseeable consequences.
Iran might also decide to retaliate by stepping up military support for Houthi Shi'ite rebels in Yemen, who are fighting a Saudi-led alliance. Coincidentally, Riyadh announced on Jan 2 that it was ending a partial ceasefire in Yemen that began last month. Hostilities are expected to intensify despite a new round of peace talks.
In addition, the latest tensions could sabotage Western attempts to get Iran and Saudi Arabia to collaborate in bringing an end to Syria's civil war. Iranian spokesmen were quick to draw a link between executions of alleged terrorists and alleged Saudi support for Salafist Sunni terror groups in Syria. Despite reports that it has recently been pulling back, Teheran still has the option of stepping up operations inside Syria by its Revolutionary Guard units and its Lebanese militia ally Hezbollah against Saudi-backed anti-government forces.
Why is there a divide between Sunni and Shi'ite Muslims?
A schism emerged after the death of the Prophet Muhammad in 632. He died without appointing a successor to lead the Muslim community, and disputes arose over who should shepherd the new and rapidly growing faith. Some believed that a new leader should be chosen by consensus; others thought that only the prophet’s descendants should become caliph.
The title was passed to a trusted aide, Abu Bakar, though some thought it should have gone to Ali, the prophet’s cousin and son-in-law. Ali eventually did become caliph after Abu Bakar’s two successors were assassinated.
After Ali was also assassinated in Kufa - in what is now Iraq - his sons Hasan and then Hussein claimed the title. But Hussein and many of his relatives were massacred in Karbala, Iraq, in 680. His martyrdom became a central tenet to those who believed that Ali should have succeeded the prophet (it is mourned every year during the month of Muharram). The followers became known as Shi'ites, a contraction of the phrase Shiat Ali, or followers of Ali.
The Sunnis, however, regard the first three caliphs before Ali as rightly guided and the true adherents to the Sunnah, or the prophet’s tradition. Sunni rulers embarked on sweeping conquests that extended the caliphate into North Africa and Europe. The last caliphate ended with the fall of the Ottoman Empire after World War I.
How do their beliefs differ?
The Sunni and Shi'ite sects of Islam encompass a wide spectrum of doctrine, opinion and schools of thought. The branches are in agreement on many aspects of Islam, but there are considerable disagreements within each. Both branches include worshipers who run the gamut from secular to fundamentalist.
Shi'ites consider Ali and the leaders who came after him as imams. Most believe in a line of 12 imams, the last of whom, a boy, is believed to have vanished in the ninth century in Iraq after his father was murdered. Shi'ites known as Twelvers anticipate his return as the Mahdi, or Messiah.
Because of the different paths the two sects took, Sunnis emphasize God’s power in the material world, sometimes including the public and political realm, while Shi'ites value in martyrdom and sacrifice.
Which sect is larger and where is each concentrated?
More than 85 per cent of the world's 1.5 billion Muslims are Sunni. They live across the Arab world, as well as in countries like Turkey, Pakistan, India, Bangladesh, Malaysia and Indonesia. Iran, Iraq and Bahrain are largely Shi'ite.
The Saudi royal family, which practices an austere and conservative strand of Sunni Islam known as Wahhabism, controls Islam's holiest shrines, Mecca and Medina. Karbala, Kufa and Najaf in Iraq are revered shrines for the Shi'ites.
Saudi Arabia and Iran, the dominant Sunni and Shi'ite powers in the Middle East, often take opposing sides in regional conflicts. In Yemen, the Houthis, Shiite rebels from the north, overthrew a Sunni-dominated government, leading to an invasion by a Saudi-led coalition.
In Syria, which has a Sunni majority, the Alawite Shiite sect of President Bashar Assad, which has long dominated the government, clings to power amid a bloody civil war. And in Iraq, bitter resentments between the Shiite-led government and Sunni communities have contributed to victories by ISIS.
SOURCE: BLOOMBERG, THE GUARDIAN, NEW YORK TIMES