There can be no peace in Syria unless Iran and Saudi roll back
It was an ominous start to the new year for an already deeply troubled Middle East. Last Saturday, Saudi Arabia announced it had executed 47 people in a single day. The mass executions were notable not just because of the high number killed, but the fact the dead included Saudi cleric Nimr al-Nimr, a prominent figure within the country's beleaguered Shia minority community.
While most likely related more to internal dynamics in Saudi Arabia than anything else, his execution caused outrage among Shia elsewhere in the Middle East and as far away as Pakistan, and proved a lightning rod in the ongoing and increasingly toxic regional power play between the kingdom and its arch nemesis, Iran.
Soon after news of Nimr's killing broke, protesters in Tehran ransacked the Saudi embassy there. The Iranian authorities moved quickly to name the street where the embassy stands after Nimr, just like they renamed the street where the British embassy is located after Bobby Sands in the 1980s.
The following day, Riyadh formally cut diplomatic relations with Iran and leaned on others - including Bahrain, the UAE, Kuwait and Sudan - to follow suit, while also threatening to sever commercial ties with Tehran. Iraq's prime minister, Haider al-Abadi, denounced Nimr's execution, warning of "repercussions" for regional security, while Iran's Revolutionary Guards said Riyadh could expect "harsh revenge".
By Thursday, Iran was accusing Saudi Arabia of launching airstrikes against its embassy in war-ravaged Yemen.
With the belligerent rhetoric continuing amid real fears of a serious escalation beyond the diplomatic, the repercussions will be felt not only in Syria and Yemen - where both countries back rival warring factions - but in the region more widely.
Riyadh and Tehran have long considered each other foes competing for influence in the Middle East, with both oil-rich states bidding to be the dominant power.
The rivalry plays out not just diplomatically or commercially but for real on the battlefields of Syria, and more recently Yemen, through the support of proxy forces, including militant groups.
Because Saudi Arabia claims to be the standard bearer for Sunni Islam and Iran the same for Shia Islam, the competition between the two powers has helped fuel sectarianism across the region.
While the schism between Sunni and Shia Islam dates back to the early years of the faith in the seventh century, the current regional violence manifesting itself along Sunni-Shia lines has less to do with a religious divide than how that historical rupture has been exploited as part of the Riyadh-Tehran struggle for power.
Amping sectarian sentiment for these purposes is not a new tactic for either Saudi Arabia or Iran.
After Iran's 1979 revolution, the Shia clerics who came to power there sought to "export" their revolutionary model, prompting Riyadh to promote a fundamentalist - and anti-Shia - interpretation of Sunni Islam in order to counter Iranian influence. The US invasion of Iraq in 2003 played to Iran's advantage, helping install its local allies there and bolstering its influence, but it also fanned Sunni extremism among those who lost out after Saddam Hussein was dislodged.
The seeds of what is now known as Isil, itself rabidly sectarian, were sown in bitter divides of post-invasion Iraq.
Today, the fault lines of the fighting in Syria and in Yemen have settled in many ways along Sunni and Shia lines, due in no small part to Riyadh and Tehran's patronage of key protagonists. The funding of extremist factions in Syria gave a more sectarian hue to what had initially been a largely non-sectarian uprising against the regime of Bashar al-Assad. There can be no peace in Syria or Yemen unless the two regional giants are prepared to roll back their support of favoured actors.
This week's escalation between Saudi Arabia and Iran could not have come at a worse time. There were tentative indications that both might have been heading for something approaching a mild rapprochement in 2016, perhaps even helping de-escalate the wars in Syria and Yemen.
Talks aimed at resolving the latter were scheduled for later this month. Efforts to address the Syria quagmire had been stepped up in recent months.
This week's events threaten to upend all that delicate diplomatic manoeuvring and raise Sunni-Shia tensions even further on the ground.