Saudi Arabia’s execution of the Shiite cleric Sheikh Nimr al-Nimr early this month has stoked tensions between it and Iran. This political confrontation has been playing out across the region as Sunnis and Shiites wage proxy wars across the Middle East and Persian Gulf. This battle will intensify as domestic pressures mount in both Saudi Arabia and Iran, and regional geopolitics become more fluid.

Saudi Arabia, a Sunni kingdom, and Iran, a Shiite Islamic republic, have contested regional leadership for centuries, a struggle that was both political and religious. Until 1979, Iran was ascendant, a position that could be credited to Tehran’s alliance with the United States. The Iranian Revolution ended that partnership, and Saudi Arabia quickly filled the gap as the closest U.S. partner among Arab states. Washington worked closely with Riyadh, and other regional governments, to contain Iranian influence.

The success of that effort prompted Tehran to sponsor proxies in such countries as Syria, Lebanon and Yemen, as well as to urge Shiite communities throughout the region to stand up against Sunni oppression. Conclusion of a nuclear agreement with the P5+1 heralds the end of Iran’s isolation and renewed efforts to extend the country’s influence in the Middle East and beyond.

The Saudi government is quick to see Tehran’s hand in any domestic protests, and al-Nimr, a Shiite cleric who championed the poor and marginalized Shiites in Saudi Arabia, was an easy target. A vocal critic of the Riyadh government, he was arrested in 2012, convicted of supporting terrorism and had been awaiting execution until Jan. 2, when, he along with 46 others, was put to death.

The killing sparked demonstrations in Tehran in which some protesters attacked the Saudi Embassy; the next day Riyadh broke diplomatic relations with Iran, followed by two other Arab countries, Bahrain and Sudan, as well as Somalia. The United Arab Emirates and Kuwait have recalled their ambassadors and downgraded diplomatic ties with Iran.

The hard line against Iran reflects the determination of Saudi Arabia’s King Salman to battle the expansion of Iranian influence. The kingdom has put together a coalition that is countering the Houthis, Shiite-backed forces, in Yemen and is backing groups battling Syrian President Bashar Assad, who enjoys Iranian support as well.

The intensified hostility between Saudi Arabia and Iran will have the most immediate effect on peace talks to address the Syrian civil war. Negotiations will soon begin in Vienna to deal with that bloody four-year conflict, and there is little chance of success if two key supporters of the fighting forces see continued utility in a proxy fight. A similar faceoff is likely over Iraq, where Iran exercises growing influence over the Baghdad government and Saudi Arabia is prepared to back increasingly embittered Sunnis.

Saudi Arabia has been unnerved by the seeming readiness of the U.S. to make common cause with Iran. Riyadh views the nuclear deal with great suspicion and worries that Washington’s priorities are shifting in an era of low oil prices and the prospect of U.S. energy independence.

Low oil prices also have a powerful impact on the Saudi economy. Reduced revenues is forcing a reconsideration of the social contract between the Riyadh government and its citizens, a process that is likely to intensify gaps between the Sunni and Shiite communities in Saudi Arabia. The violence in Iran will embolden Saudi hard-liners who insist the Shiites at home and abroad are a threat and must be dealt with accordingly. Saudi Foreign Minister Adel Al-Jubeir accused Iran of having a “hostile policy,” and of smuggling arms and creating terrorist cells throughout the region. He promised that Saudi Arabia will not allow Iran “to undermine our security.”

Hard-liners in Iran will be similarly emboldened. Supreme leader Ayatollah Ali Khamenei warned of “divine revenge” over al-Nimr’s death and the protesters no doubt saw their actions as sanctioned by the highest authorities. Iran will hold elections for the consultative assembly, while a new Assembly of Experts (which approves the selection of the supreme leader) will also be picked. New tensions with Saudi Arabia can shift the balance of power between moderates and conservatives in those campaigns.

Cognizant of the risk to his own power, Iranian President Hassan Rouhani condemned both al-Nimr’s execution and the violence against the Saudi Embassy. Rouhani, a moderate, has pushed reconciliation with the West and knows that intensified hostility with Saudi Arabia threatens to undo his recent accomplishments.

If the leaders in Saudi Arabia and Iran continue to view their relationship in zero-sum terms, there will be more crises. Sunnis and Shiites are scattered throughout the region, and suspicions and hostility have long dominated perceptions of the other. With governments in Iraq, Yemen and Syria struggling to hold their embattled countries together, there will be ample opportunities for mischief making — or worse. More troubling still, there is no country that can act as peacemaker.

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