(CNN) -- They came in their thousands. Old women in chador, zealous young men, in battered minibuses and family cars: Shia from Iran on a pilgrimage to the Askari mosque in Samarra, a city some 80 miles north of Baghdad.
It was January 2004. The golden dome of the Askari mosque, one of the most important shrines to Shiite Muslims, gleamed above the crowded streets of Samarra. On the city's outskirts, Iranian families stopped for picnics.
Nearly a year after Saddam Hussein was ousted, Iranian pilgrims were flocking to cities in Iraq, able at last to visit their faith's holiest places.
The scene in Samarra was a dramatic symbol of the new order in Iraq, and a further sign of the Shiite resurgence that had begun with the Iranian revolution in 1979 and the rise of Hezbollah in Lebanon in the early 1980s.
And shrines matter. The scholar Vali Nasr later wrote that "hundreds of thousands of pilgrims, coming from countries ranging from Lebanon to Pakistan, have visited Najaf and other holy Shiite cities in Iraq, creating transnational networks of seminaries, mosques, and clerics that tie Iraq to every other Shiite community, including, most important, that of Iran."
Nasr added this cautionary note: "Stemming adversarial sectarian politics will require satisfying Shiite demands while placating Sunni anger and alleviating Sunni anxiety, in Iraq and throughout the region."
The opposite has happened, providing fertile ground for the Islamic State in Iraq and Syria (ISIS) -- a fundamentalist Sunni group with a visceral hatred for the Shiite majority. Its offensive is a defining moment in the age-old enmity of Sunni and Shiite Muslims in Iraq -- and will have consequences far beyond Iraq -- in Syria, Iran and the Gulf states.
Ian Bremmer, President of Eurasia Group, a political risk consultancy, says: "Borders are likely to be redrawn through the bloodshed. Those borders may ultimately prove more durable, yes, but they could also provoke interstate war across the region. And it's the latter outcome that makes Iraq so important."
Symbols and shrines
In images posted Sunday and purportedly showing captured Iraqi soldiers being marched to their execution, ISIS described the prisoners as "apostates heading to their hole of doom." Another video showed a brutal inquisition of several captured soldiers, with one man saying: "These are Maliki's dogs. These are Maliki's soldiers and we are the soldiers of God."
ISIS seems determined to enrage Shiites and bring on a religious war, which makes Samarra a likely target. (The city was also the birthplace of ISIS' leader Abu Bakr al Baghdadi.) The Askari mosque was bombed by ISIS' precursor, al Qaeda in Iraq, in February 2006: it was believed to be a calculated attempt by the group's leader at the time, Abu Musab al Zarqawi, to ignite a regional sectarian conflict. The bombing sparked retaliatory attacks in which thousands of Sunnis were killed.
Acknowledging its importance, Iraq's Prime Minister Nouri al-Maliki, a Shiite, went to Samarra last week to inspect its defenses, and said on Saturday: "Samarra will be the starting point, the gathering station of our troops to cleanse every inch that was desecrated by footsteps of those traitors."
Grand Ayatollah Ali al-Sistani, Iraq's foremost Shiite religious leader, called for shrines to be defended.
"Citizens who are able to bear arms and fight terrorists... should volunteer and join the security forces to achieve this holy purpose," Sistani said through a representative. Iraqi Shiites - young men and old -- responded in the thousands.
With every passing day, the rhetoric in Iraq -- on both sides -- takes on a sharper sectarian tone.
The Institute for the Study of War, which closely follows the Iraqi conflict, says that so far Shiites have shown limited appetite for revenge killings. "However, the ISIS threat to the [Askari] shrine will unravel that restraint and trigger retaliatory attacks against the Iraqi Sunnis," the Institute says. "Destruction of any of these shrines would bring on full scale ethno-sectarian civil war with violence meeting or surpassing 2006 levels."
A long and bitter history
The Sunni-Shiite divide began in the seventh century, essentially over the legitimacy of the successors of the Prophet Mohammed. One of its most fateful battles took place in Karbala (now an Iraqi city) in 680 AD, when the army of Hussein, Mohammed's grandson, was defeated and massacred by the Sunni Caliph, and he was beheaded. Shiites still mark Hussein's death every year in the ritual of Ashura.
The sectarian fault-line has since caused many more massacres and institutionalized repression, in contemporary Iraq and beyond. The Shiites have seen themselves as an oppressed majority or minority in places such as Lebanon, Bahrain, Saudi Arabia, parts of Pakistan and Afghanistan -- and Iraq.
In the aftermath of the first Gulf War in 1991, and the eviction of Iraqi forces from Kuwait, Shiites in southern Iraq revolted against Saddam Hussein. Clinging to power and with his army in disarray, Saddam ordered the brutal suppression of the uprising. The Republican Guard carried out mass executions, even tying civilians to the front of their armored vehicles. Tens of thousands of Shiites fled into the marshes around Basra, and were hunted down over the following months.
Their nemesis would finally be routed 12 years later. The U.S.-led Operation Iraqi Freedom brought a sense of liberation to the Shiites, but the new order was a threat to Sunnis. Newly-formed political parties largely followed sectarian lines.
Over the past three years, the Maliki government has played to its Shiite base and deepened the resentment of Sunni tribes, who believe they have been victimized and starved of resources. Rep. Adam Schiff, a senior member of the U.S. House Intelligence Committee, puts it this way: "[Maliki] has created a government of the Shiite, for the Shiite and by the Shiite -- and until that changes, he will increasingly push the Sunni population into the arms of extremist groups" like ISIS.
To many observers the turning point was the arrest last December of a prominent politician in Ramadi, the Sunni heartland, and a military operation against an anti-government protest camp in the city. Maliki had described it as an al Qaeda base, infuriating local sheikhs. After ISIS took control of much of the Sunni city of Fallujah in January, Iraqi airstrikes further alienated local people.
It is difficult to see how ISIS could have made such stunning advances without at least tacit support from the Sunni tribes. Zarqawi's AQI over-reached with its arbitrary and vicious punishment of Sunnis who failed to accept its militant Salafism. But ISIS, despite quickly declaring Sharia law in towns and cities it has taken, does not appear to have alienated mainstream Sunnis -- yet -- and has instead focused its brutality on Shiites.
A common enemy brings Iraq and Syria closer...
In a clever strategic move, ISIS began infiltrating northern Syria early in 2013. The benefits were many. It bought strategic depth for the group and allowed it to set up hubs in places like Raqqa while the attention of Syrian President Bashar al-Assad's regime was elsewhere. ISIS was also able to sharpen the sectarian nature of the Syrian conflict, attracting foreign fighters by the hundreds who could easily cross the border from Turkey.
The group's exponential growth in both Syria and Iraq has set the stage for an upheaval that could erase colonial-era borders and divide the region along sectarian lines.
Faced with this threat, there are already signs of new co-operation between Maliki's government and the Assad regime in Syria, itself largely made up of Alawites, a Shiite sect. The Syrian Observatory for Human Rights reported Sunday that the Syrian air force had begun pounding ISIS bases "including those in the northern province of Raqqa and Hasakeh in the northeast." The Observatory said the "intense" attacks were being carried out "in coordination with the Iraqi authorities."
The Iraqi government would clearly welcome any disruption of ISIS' supply lines or communications. The Syrian attacks are not altruistic; the Assad regime is likely responding to reports that ISIS is bringing some of the heavy weaponry it has captured in Iraq back across the border.
... makes Iran more powerful ...
The Iraqi Prime Minister is also more reliant than ever on support from Iran, given the way that the Iraqi Security Forces have crumbled. U.S. officials say the powerful head of Iran's elite Quds Force, General Qassim Suleimani, was in Iraq last week, amid reports that Iranian militia were fighting alongside the remnants of the Iraqi army. Iran has also sent roughly 500 Revolutionary Guard troops to fight alongside Iraqi forces in Diyala province, a senior security official in Baghdad told CNN.
The Institute for the Study of War concludes "it is likelier now ... that a Shiite unity government more dependent on Iran will emerge in Baghdad if the ISIS crisis is averted."
The Obama Administration suddenly finds itself on the same side as Iran in trying to prevent a terror group more extreme and more effective than al Qaeda from overrunning half of Iraq. And U.S. Secretary of State John Kerry has not closed the door to co-operation with the Islamic Republic.
"Let's see what Iran might or might not be willing to do before we start making any pronouncements," he told Yahoo News on Monday. "I think we are open to any constructive process here that could minimize the violence, hold Iraq together ... and eliminate the presence of outside terrorist forces that are ripping it apart."
Syria's government has also sought reinforcements from Iran and the Lebanese Shiite militia Hezbollah to try to reverse rebel gains (most of them made by Sunni Islamist groups.) Senior advisers from the Quds Force of the Iranian Revolutionary Guard have been working with the Syrians, according to multiple reports from the past year, and Iranian weapons have been flown into Damascus.
... and threatens to drag in the Gulf states
A decisive shift in Iraq's place in the Middle East, either by its division or a much closer reliance on Tehran, is sending shivers through the largely Sunni Gulf states. Saudi Arabia lambasted Iraq's Maliki on Monday, with Information Minister Abdulaziz Khoja saying the crisis would have been averted "if it wasn't for the sectarian and exclusionary policies that were practiced in Iraq in recent years." Qatar has expressed a similar view.
Maliki's office shot back, blaming Saudi Arabia for supporting extremist groups and adding: "The Saudi government must bear responsibility of the serious crimes committed by these groups."
As the struggle unfolds, there is probably one thing on which all parties in the region would agree, from Bashar al-Assad in Syria to ISIS leader Abu Bakr al Baghdadi to Grand Ayatollah Sistani: they are all now part of an existential struggle in the heartland of Islam.