Protesters in Beirut display images of Sheikh Nimr al-Nimr, whose execution, by Saudi Arabia, set off violent protests across the Middle East. PHOTOGRAPH BY HASAN SHAABAN / REUTERS

The rule of thumb in the Middle East is that diplomacy often—too often—makes progress only to be overtaken by unforeseen violence on the ground. It’s happening again. Tensions between the Islamic world’s rival powers—the Sunni monarchy of Saudi Arabia and the Shiite theocracy in Iran—that erupted over the New Year’s weekend now jeopardize a string of fragile peace initiatives: Peace talks on Syria (the political complement to the military campaign against the Islamic State) are set to begin January 25th. The Iran nuclear deal was expected to be implemented this month. Iraq is trying to consolidate its first military and political gains against ISIS, which were achieved last month. And a three-week ceasefire in Yemen’s ruthless civil war collapsed on January 2nd, endangering a second round of peace talks scheduled for this month. These initiatives are essential to the international effort to reconstruct the disintegrating map of the Middle East.

Saudi Arabia and Iran—and their allies—are pivotal players in each flashpoint. Both countries have to make concessions for diplomacy to succeed anywhere. But, on January 3rd, Riyadh abruptly severed diplomatic relations with Tehran.

The drama began on January 2nd, when executioners in a dozen Saudi cities carried out death sentences—by beheading or firing squad—on forty-seven men. It was the largest mass execution since sentences were carried out against extremists who seized the Grand Mosque in Mecca, more than thirty-five years ago. This time, many of the men were convicted of having links to Al Qaeda, whose agenda was not all that different from that of the earlier extremists. Most were Sunni.

But among them was Sheikh Nimr al-Nimr, a charismatic cleric with no connections to Al Qaeda, who had championed the long-oppressed Shiite minority in Saudi Arabia. Shiites make up between ten and fifteen per cent of Saudi Arabia’s twenty-seven million people. They live and work in oil-rich areas of the country but have long felt they aren’t getting a fair share of their own resources. For decades, there were sporadic protests among Shiites in the eastern provinces, over economic and political grievances. Nimr was arrested in 2012, after a round of protests spawned by the Arab Spring. He was beheaded on Saturday for “breaking allegiance with the ruler,” instigating unrest, and “undermining the kingdom’s security.”

At a meeting with U.S. officials in 2008, Nimr insisted that he was not as radical as he had been portrayed publicly, according to a diplomatic cable released by Wikileaks. He told the Americans that he strongly opposed the “authoritarianism of the reactionary Saudi regime” and, in any future conflict, would stand with the people, rather than with the government. But he “sought to distance himself from pro-Iranian and anti-American statements,” the officials reported to the State Department and U.S. intelligence agencies, and noted that he “espoused other conciliatory ideas such as fair political decision-making over identity-based politics, the positive impact of elections, and strong ‘American ideas’ such as liberty and justice.” The cable noted the Sheikh’s growing appeal among the young, the lower classes, and those disaffected by discrimination and the kingdom’s economic malaise.

Nimr’s execution sparked immediate protests among Shiite communities from Turkey to Lebanon, Bahrain, Pakistan and northern India—and in Dearborn, Michigan, and New York City. At a demonstration at Columbus Circle on Sunday, protesters carried a sign comparing Nimr to Martin Luther King, Jr.

The protests turned violent in Iran, where the government stoked passions about injustice and martyrdom, concepts at the core of the Shiite faith and the split with the dominant, Sunni branch of Islam, shortly after the Prophet Muhammad’s death. Relations had been deteriorating since September, when hundreds of Iranian pilgrims were among two thousand crushed to death during the annual hajj pilgrimage. Iran accused the Saudis of mismanagement and abuse; for months, the Saudis did not return many of the bodies.

After Nimr’s execution, Iran’s Supreme Leader, Ayatollah Ali Khamenei, warned that “divine retribution” would “grip the neck of Saudi politicians.” Within hours, mobs attacked and set fire to the Saudi Embassy in Tehran and ransacked the consulate in Mashhad. Saudi Arabia charged that repeated appeals to Tehran to rein in the crowd were ignored. On Sunday, Tehran arrested forty protesters. “We do not allow rogue groups to commit illegal actions and damage the holy reputation of the Islamic Republic of Iran,” President Hassan Rouhani pledged. But it was too late, and the Saudis broke off diplomatic relations.

“The history of Iran is full of negative and hostile interference in Arab countries, always accompanied by ruin, destruction, and the killing of innocent souls,” the Saudi Foreign Minister, Adel al-Jubeir, told a hastily convened press conference. Complicating matters, in 2011, when Jubeir was the Saudi Ambassador to Washington, he was the target of a bizarre assassination plot by an Iranian-American used-car salesman, Manssor Arbabsiar, who tried to hire a Mexican drug cartel to bomb a restaurant where Jubeir dined. Arbabsiar admitted to plotting with members of the Iranian military. In 2013, he was sentenced in a Manhattan court to twenty-five years. The animosity has a personal as well as a professional dimension among the current crop of politicians in both capitals.

On Monday, Bahrain and Sudan also severed relations with Iran, and the United Arab Emirates downgraded ties. The regional fallout is probably not over.

The outside world, while faulting the Saudis for initiating the current crisis, condemned actions by both Riyadh and Tehran. In an unusually candid statement on Saturday, U.N. Secretary-General Ban Ki-moon said that he was “deeply dismayed” by Nimr’s execution, and questioned “trials that raised serious concerns over the nature of the charges and the fairness of the process.” The European Union called into question “freedom of expression and the respect of basic civil and political rights” in Saudi Arabia. Jubeir rebuffed criticism of the executions. “We should be applauded for this, not criticized,” he said Monday, in an interview with Reuters.

The United States is now scrambling to contain the damage, even as U.S. officials are alarmed by Saudi Arabia’s provocative act. “We are particularly concerned that the execution of prominent Shia cleric and political activist Nimr al-Nimr risks exacerbating sectarian tensions at a time when they urgently need to be reduced,” the State Department spokesman John Kirby said. “The United States also urges the government of Saudi Arabia to permit peaceful expression of dissent and to work together with all community leaders to defuse tensions in the wake of these executions.‎” From his vacation, Secretary of State John Kerry talked with Iranian Foreign Minister Javad Zarif on Sunday, and on Monday he spoke by telephone with Saudi and other Arab leaders to prevent the crisis from worsening. Russia also offered to intervene with both countries, while the U.N. special envoy for Syria set off for meetings in both capitals to salvage the Syria peace talks.

The last break in relations between Saudi Arabia and Iran, also initiated by Riyadh, occurred in 1988. It lasted three years. The current split mirrors a fundamental ideological and strategic division across the Middle East that is now at least as significant as the Arab-Israeli divide, which defined Mideast conflicts over the past six decades. The escalating sectarian rift in recent years is also one of the deepest fractures since the original schism between Sunnis and Shiites, nearly fourteen centuries ago, shortly after the death of the Prophet Muhammad. Then there are the ethnic tensions, dating back centuries, between Arabs and Persians. Even if the United States and others succeed in limiting the damage of the immediate crisis, prospects for healing the deeper divide seem more than unlikely.