Saudi Arabia’s execution on Jan. 2 of 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. Demonstrators stormed the Saudi embassy in Tehran 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 Jan. 3, the kingdom cut off diplomatic relations with Iran, a move followed by several Saudi allies. How did the execution of a cleric escalate so quickly into a diplomatic crisis between two regional rivals that have been fighting a cold war for over a decade? Saudi leaders have been dismayed since July, when the United States and five other world powers reached an agreement with Iran to limit its nuclear program in exchange for lifting international sanctions. Under the deal, Iran will be allowed to re-enter the global financial system, increase its oil exports and access more than $100 billion in frozen assets. Saudi Arabia is worried that the nuclear deal will help Iran gain an edge in their ongoing regional rivalry. This series of proxy battles — in which the two rivals are backing competing factions in Iraq, Syria, Yemen, Lebanon and Bahrain — have shaped the Middle East since the United States invaded Iraq in 2003. While the conflict is partly rooted in the historical 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. These proxy wars, which involve other powers aside from Iran and Saudi Arabia, are at the root of much of the destruction in the region over the past five years. They have cost hundreds of thousands of lives, especially in Syria, where more than 250,000 have been killed since the March 2011 uprising against the regime of Bashar al-Assad. The Syrian war has also spurred the most severe refugee crisis since World War Two, with nearly 4.4 million Syrians forced to seek refuge in neighboring countries. In late March, Saudi Arabia launched a war against Houthi rebels and their allies in Yemen. The Houthis, who belong to a sect of Shi’ite Islam called Zaydis, are allies of Iran. While the Saudis are quick to label the Houthis as Iranian proxies, it’s unclear how much support they actually receive from Tehran. As the war has dragged on, air strikes by Saudi Arabia and its Sunni allies caused most of the estimated 2600 civilian deaths. The wars in Syria and Yemen can’t be stopped without an agreement between Iran and Saudi Arabia. With the two rivals now refusing to speak to one another, the violence is likely to get worse. While Saudi officials insist they will not boycott United Nations-sponsored peace talks on Syria, scheduled for Jan. 25, the prospects for a breakthrough are slim. By default, more bloodshed and chaos will strengthen the militants of Islamic State, who have established their self-declared capital in the Syrian city of Raqqa. In Yemen, al Qaeda in the Arabian Peninsula and other militant groups are taking advantage of the power vacuum. 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 ultraconservative Wahhabi clerics who support them, construe political legitimacy on the medieval 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 sometimes-restive Shi’ite minority, which is concentrated in the Eastern Province, where most of the kingdom’s oil reserves lie. The executed cleric, Sheikh Nimr al-Nimr, was a leader of the Shi’ite community, which makes up about 10 percent of the kingdom’s 20 million citizens. At times, he advocated secession for parts of the oil-rich Eastern Province. More broadly, since the US invasion of Iraq, the traditional centers 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 Hezbollah and Hamas, and its alliance with Syria. (In Iraq, Saudi Arabia backed Sunni insurgents, while Iran supported the new Shi’ite-led government and militias.) After the Iraq invasion, Sunni Arab leaders were largely silent in public, afraid of antagonizing Iran or appearing too close to the unpopular Bush Administration. But in private, they wanted someone else to take care of their “Iran problem,” and by 2007, they were clamoring for the United States or Israel to attack Iran’s nuclear installations, no matter the consequences. One secret US diplomatic cable released by WikiLeaks detailed a series of meetings in April 2008 between then-King Abdullah, senior Saudi princes and top US officials who were in Riyadh to discuss American policy in Iraq. Saudi leaders were furious over Iranian influence in Iraq, and their inability to counter it. The then-Saudi Ambassador to Washington, Adel al-Jubeir (who is now foreign minister), repeated “the King’s frequent exhortations to the US to attack Iran and so put an end to its nuclear weapons program.” The king, Jubeir scolded one American diplomat, “told you to cut off the head of the snake.” The Saudi conflict with Iran intensified after the Arab uprisings of 2011, when the House of Saud tried to choke off revolutionary momentum in the region. The Saudi regime became especially nervous when the revolutions spread to Yemen, on its southern border, and Bahrain, a Shi’ite-majority country ruled by a Sunni monarchy only 16 miles 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. And the proxy war quickly expanded to Syria. Soon after the peaceful demonstrations in Syria turned violent, in response to Assad’s ruthless crackdown, Saudi Arabia, Qatar and Turkey began sending money and weapons to the rebels. Iran provided funds, arms and military advisers to prop up Assad’s regime. The Syrian uprising did not start out as a religious battle, but it quickly took on sectarian overtones and descended into civil war. Both powers increasingly view their rivalry as a winner-take-all conflict: if the Shi’ite Hezbollah gains an upper hand in Lebanon, then the Sunnis of Lebanon — and by extension, their Saudi patrons — lose a round to Iran. If a Shi’ite-led government solidifies its control of Iraq, then Iran will have won another round. So the House of Saud rushes to shore up its allies in Yemen, Syria, Bahrain and wherever else it fears Tehran’s influence. As long as Iran and Saudi Arabia see their rivalry as a zero-sum game — where one can only gain at the expense of the other — the Middle East will be plagued with sectarian conflict. Mohamad Bazzi is a journalism professor at New York University and former Middle East bureau chief at Newsday. A former fellow at the Council on Foreign Relations, he is writing a book on the proxy wars between Saudi Arabia and Iran