Built on a mound overlooking the Khasa River, the walled citadel stands as a shattered symbol of a bygone era in Iraq, one in which different religious and ethnic groups coexisted. The site contains the Green Dome Mosque, the Red Church, and a tomb that some locals say is that of the Jewish prophet Daniel.

“History dictates there should be bonds of brotherhood between all the peoples of this city: Arabs, Turkmen, Kurds and Assyrians,” says Youssef, a former army general visiting the ancient site with his two wives and their children.

Such aspirations have frequently run afoul of Iraq's zero-sum politics in which one group's gain is another's loss. In Kirkuk, a disputed, oil-rich city that recently changed hands, these tensions are never far from the surface. And, as Iraq wrestles with the existential challenge of another Sunni uprising, the city's new rulers are meeting a reception that may point to a bumpy path ahead.

On his sunset stroll through the citadel, Youssef, a Sunni Arab who declined to give his full name, said security had improved since Kurdish forces established control over the city after the Iraqi army fled ahead of a Sunni Arab militant advance.

“Security is excellent. Geography bestowed the responsibility of securing Kirkuk on the Kurdish Regional Government,” he says, referring to the semi-autonomous Iraqi Kurdish administration.

That geography is key: Kirkuk, capital of a province of the same name, lies outside the current Kurdish line of control, which is a legacy of a 1991 standoff between Kurdish forces and former dictator Saddam Hussein's army. Kurdish leaders have long sought to incorporate the city – the "Kurdish Jerusalem" in their telling – into their homeland. But that ambition met stiff opposition from the province’s Arab and Turkmen residents as well as the central government. Now the city has fallen to Kurdish peshmerga fighters.

While Youssef and other non-Kurds here are pragmatic about Kurdish control, their country's unraveling is another matter.

“Separation is not in the interest of Iraq,” he says. “We need a government that respects all the people of Iraq. At the moment the Kurds are offering a service by securing the city perimeters."

Demographic shifts

Kirkuk's inter-communal tensions are rooted in conflicting historical narratives that span thousands of years and track demographic shifts in detail. Modern events have only added fuel to the fire.

In the 1990s, Saddam Hussein ordered renovations of Kirkuk's walled citadel that were used as cover to evict some 900 Turkmen and Kurdish families from their homes. The evictions were part of a broader policy of "Arabization" in a number of northern cities with large ethnic-minority populations.

Since the 2003 US-led invasion that toppled Saddam, the trend has reversed: Kurds are now in the majority in Kirkuk; Arabs, Turkmen, and Assyrians – also known as Chaldean Christians – are minorities, though the exact makeup is unknown since the last full census was held in 1957.

As elsewhere in Iraq, Kirkuk’s Christians are in retreat. Narzi Toma, an Assyrian priest, says his parish has shrunk from 800 worshippers in the 1990s to roughly 300 today.

Hundreds of thousands of Christians have fled Iraq since 2003, driven away by insecurity and the rise of radical groups, such as the Islamic State of Iraq and the Levant (ISIS) that has been instrumental in the current Sunni Arab offensive.

For Mr. Toma, Kurdish control over Kirkuk is far more palatable than rule by Sunni extremists like ISIS, which has targeted Christian places of worship and clergy.

“If the Kurds are good for us Christians, then we want to be with them but in the end the decision falls to politicians,” says the priest. “The biggest threat that we face is ISIS. They kill anyone who is different.”

Turkmen discontent

The Turkmen community, considered to be Iraq’s third largest ethnic group, are less thrilled about the prospect of being under Kurdish control.

“The security situation in Kirkuk is not good because only one power is in control. I don’t feel safe in my city,” says Ali Shokur, a lawyer and community leader, during an informal gathering of Turkmen activists and personalities.

His friend, Mofaq Beyoglu, a member of the Iraqi Turkmen Front, an umbrella group, said that power should be shared between communities. “This city with all its ethnicities should have a joint administration and not be ruled by just one party because this will result in instability and insecurity,” he says.

Back at the citadel, a sprightly old man tends to visitors at the tomb attributed to the prophet Daniel. It lies under a mosque that was originally a Jewish synagogue then a Christian church. In the eyes of Faruk Mohammed Saleh, the citadel is a shining example of tolerance in Kirkuk.

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“There is no distinction between the prophets,” he says repeatedly, referring to the Quranic passages that acknowledge the Hebrew and Christian Bibles.

Mr. Saleh’s family has lived in the area for generations. He takes great pride in caring for the site and the adjacent cemetery where his grandfather is buried. But his political views, and his vision for Iraq, isn't up for discussion. “Only God knows. I just want security to improve so more people visit.”