Behind much of the Middle East's chaos - the wars in Syria and Yemen, the political upheaval in Iraq and Lebanon and Bahrain - there is another conflict. Saudi Arabia and Iran are waging a struggle for dominance that has turned much of the Middle East into their battlefield. Rather than fighting directly, they wield and in that way worsen the region's direst problems- dictatorship, militia violence and religious extremism.
The history of their rivalry tracks - and helps to explain - the Middle East's disintegration, particularly the Sunni-Shia sectarianism both powers have found useful to cultivate. It is a story in which the US has been a supporting but constant player, most recently by backing the Saudi war in Yemen, which kills hundreds of civilians. These dynamics, scholars warn, point toward a future of civil wars, divided societies and unstable governments.
Saudi Arabia, a young country pieced together only in the 1930s, has built its legitimacy on religion. By promoting its stewardship of the holy sites at Macca and Madina, it could justify its royal family's grip on power.
Iran's revolution, in 1979, threatened that legitimacy. Iranians toppled their authoritarian government, installing Islamists who claimed to represent "a revolution for the entire Islamic world," said Kenneth M. Pollack, a senior fellow at the Brookings Institution.
The revolutionaries encouraged all Muslims, especially Saudis, to overthrow their rulers as well. But because Iran is mostly Shia, they "had the greatest influence with, and tended to reach out to, Shia groups," Pollack said.
Some Saudi Shia, who make up about 10% of the population, protested in solidarity or even set up offices in Tehran - stoking Saudi fears of internal unrest and separatism. This was the opening shot in the sectarianization of their rivalry, which would encompass the whole region.
They found that way the next year, when Saddam Hussein's Iraq invaded Iran, hoping to seize oil-rich territory, Saudi Arabia backed the Iraqis to the hilt because they want the Iranian revolution stopped.
The war, over eight years of trench warfare and chemical weapons attacks, killed perhaps 1m people. It set a pattern of Iranian-Saudi struggle through proxies, and of sucking in the US, whose policy is to maintain access to the vast oil and gas reserves that lie between the rivals.
That sense of insecurity would later drive Iran's meddling abroad, said Marc Lynch, a political scientist at George Washington University, and perhaps its missile and nuclear programs.
The 1990s provided a pause in the regional rivalry, but also set up the conditions that would allow it to later explode in such force. Saudi Arabia, wishing to contain Iran's reach to the region's minority Shia populations, sought to harden Sunni-Shia rifts. Government programmes promoted "anti-Shia incitement in schools, Islamic universities, and the media," Toby Matthiesen, an Oxford University scholar, wrote in a brief for the Carnegie Endowment.
These policies cultivated sectarian fears and sometimes violence that would later feed into the ideology of the IS.
In 1990, Iraq invaded Kuwait, a Saudi ally. The US, after expelling the Iraqis, established military bases in the region to defend its allies from Iraq. This further tilted the regional power balance against Iran, which saw the US forces as a threat.
Though it was not obvious at the time, Iraq had become a powder keg, one that would ignite when its government was toppled a decade later.
The 2003 US-led invasion, by toppling an Iraqi government that had been hostile to both Saudi Arabia and Iran, upended the region's power balance.
Iran, convinced that the US and Saudi Arabia would install a pliant Iraqi government, and remembering the horrors they had inflicted on Iran in the 1980s - raced to fill the postwar vacuum. Its leverage with Shia groups, which are Iraq's largest demographic group, allowed it to influence Baghdad politics.
Saudi Arabia sought to match Iran's reach but, after years of oppressing its own Shia population, struggled to make inroads with those in Iraq.
As their competition in Iraq heated up, Saudi Arabia and Iran sought to counterbalance each other through another weak state- Lebanon.
Lebanon provided the perfect opening- a frail democracy recovering from civil war, with parties and lingering militias primarily organized by religion. Iran and Saudi Arabia exploited those dynamics, waging a new kind of proxy struggle "not on conventional military battlefields," Gause said, but "within the domestic politics of weakened institutional structures."
Iran, for instance, supported Hezbollah, the Shia militia and political movement, which it had earlier cultivated to use against Israel. Riyadh, in turn, funnelled money to political allies such as the Sunni prime minister, Rafik Hariri.
Another political crisis, in 2008, culminated with Hezbollah overpowering Sunni militias to seize much of Beirut. Saudi Arabia requested US air cover, according to a WikiLeaks cable, for a Pan-Arab force to retake the city. Though the intervention never materialised, the episode was a dress rehearsal for the turmoil that would soon come to the wider region.
When the Arab Spring toppled governments across the Middle East, many of them Saudi allies, Riyadh feared that Iran would again fill the vacuums. So it rushed to close them, at times with force. It promised billions in aid to Jordan, Yemen, Egypt and others, often urging those governments to crack down. After pro-democracy protesters rose up in Bahrain, a Saudi ally whose Sunni king rules over a majority Shia population, Saudi Arabia sent 1,200 troops. In Egypt, Saudi Arabia tacitly supported a 2013 military takeover, seeing the military as a more reliable ally than the elected Islamist government it replaced. As Libya fell into civil war, it backed a hard-line general who was driving to consolidate control.
Though Iran has little influence in either country, Saudi Arabia's fear of losing ground to Iran made it fight harder to retain influence wherever it could, analysts believe. Syria, an Iranian ally, reversed the usual dynamic. Saudi Arabia and other oil-rich Sunni states steered money and arms to rebels, including Sunni Islamists. Iran intervened in turn, sending officers and later Hezbollah to fight on behalf of Syria's government, whose leaders mostly follow a sect of Shiism.[caption id="attachment_35870" align="aligncenter" width="800"] A female supporter of Iran's regime holds up a portrait of Iranian Supreme Leader Ali Khamenei, right, and founder of the Islamic Republic Ayatollah Ruhollah Khomeni, as demonstrators take to the streets in Tehran on December 30, 2009 in a show of force against the opposition which it accuses of being "pawns of the enemies." AFP[/caption]
The US has struggled to restore the region's balance. President Barack Obama has urged Iran and Saudi Arabia "to find an effective way to share the neighbourhood and institute some sort of cold peace," he told The Atlantic. But Lynch called this plan for "a self-regulating equilibrium" between the Mideast powers "far-fetched."
Then came Yemen. A rebel group with loose ties to Iran ousted the Saudi-backed president, deepening Riyadh's fears. Saudi Arabia launched a bombing campaign that inflicted horror on civilians but accomplished little else.
The assault receives heavy US support, though the US has few interests in Yemen other than counter-terrorism and sometimes criticises the campaign. In exchange, Riyadh acquiesced to the Iran deal and began to follow Washington's lead on Syria. But the underlying proxy war remained.[caption id="attachment_35871" align="aligncenter" width="800"] A handout picture provided by the Saudi Royal Palace on October 4, 2016 shows Saudi King Salman bin Abdulaziz,left, arriving to welcome the President of the Republic of Maldives upon the latter's arrival to Riyadh. AFP[/caption]
Asked when the Iran-Saudi struggle might cool, Pollack said he doubted that it would, "Where we're headed with the Middle East is the current trend extrapolated, with more failed and failing governments."
In Yemen, this is "reorganising Yemeni society along sectarian lines and rearranging people's relationships to one another on a non-nationalist basis," Farea al-Muslimi, an analyst, wrote in a Carnegie Endowment paper, which cited similar trends across the region.
President-elect Donald Trump will enter office having echoed Saudi Arabia's view of the region. Iran "took over Iraq," he said at a rally in January. "They're going to have Yemen. They're going to have Syria. They're going to have everything."
Mentioning both the president-elect and Hillary Clinton, Gause said he doubted that any administration could reset the Middle East's power struggles. "I do not think that the fundamental problem of the region," he said, "is something that either Trump or Clinton could do that much about."