Saudi Arabia's execution of the controversial cleric Nimr al-Nimr might well have been a domestic affair had it not involved deep sectarian schisms and regional manouevrings in the Middle East.
Sheikh Nimr had been outspoken in his defence of minority Shi'ite interests in the Sunni-led oil kingdom, had threatened to lead Shi'ites to secession and had called for the overthrow of the monarchy. His execution as a terrorist resulted in the storming of Saudi diplomatic premises in Shi'ite-led Iran amidst an outpouring of support for a man who was seen as a religious leader and not a terrorist. Iran's Supreme Leader warned of "divine revenge" against Saudi Arabia.
Riyadh cut diplomatic ties with Teheran following the sacking of its embassy and a consulate, acts that the host government was unable to prevent. Other Sunni-led countries such as Bahrain, Sudan and the United Arab Emirates have severed or downgraded relations with Iran in a sectarian war of nerves that has spilled over into the diplomatic sphere. The question is whether the state-to-state conflict will stop there or spiral into something worse.
The signs are not good. The fall in oil prices has affected Saudi Arabia's generous welfare programmes and created concerns about its record of social and political stability. Iran's rapprochement with the West has galvanised hardliners who wish to put the reformist leadership under pressure. Domestic forces in both countries make it difficult for the two governments to step back from nationalist postures.
More than this, however, it is regional politics that makes relations between them fundamentally difficult. Traditionally, Riyadh's self-definition as the leader of the Sunni Muslim world has pitted it against Teheran's claim to the mantle of Shi'ite leadership. Compounding the competition is the historical legacy of Arab-Persian rivalry, which reverberates in the region to this day. Each side sees the other as trying to expand its regional influence through sectarian appeals, or as using foreign policy to export domestic problems away.
In the latest manifestation of the underlying quest for regional dominance, Teheran supports the regime in Syria, while Riyadh backs rebel forces. In Yemen, a Saudi-led coalition is battling Shi'ite insurgents. Iran's potential to emerge as a stronger regional power, following its nuclear deal with world powers, has deepened Saudi Arabia's determination to prevent the Iranians from expanding their capacity to set the regional agenda.
Even the slim chances of a political solution to the Syrian civil war will be the first casualty of the latest crisis. It will also embolden the Islamic State in Iraq and Syria to step into the sectarian divide. Both Riyadh and Teheran need to recognise that sanity calls for de-escalating a situation that could lead to more conflict in the Middle East.