SINCE the Iraqi army and its allies began their counterattack against Islamic State (IS) in late 2014, they have managed to liberate many cities in northern and western Iraq. So it might be tempting to view the battle for Mosul, which started in the early hours of October 17th, as just one more skirmish in the jihadists’ steady retreat. But the struggle for Mosul is concentrating minds like no other encounter with IS. Why do so many inside and outside Iraq consider Mosul a turning point?

Mosul has formed the centrepiece of IS’s ambitions since its capture by the jihadist group in June 2014. From the pulpit of the city’s great mosque, Abu Bakr al-Baghdadi, the leader of IS, proclaimed himself caliph and turned Iraq’s second city into his base. In history, size and strategic location, the city dwarfed IS’s other holdings, including Raqqa, its capital in Syria. Tens of thousands of Sunni Iraqis left camps for the internally displaced to seek refuge under his rule. Without Mosul, IS will be shorn of its tax base and oil fields; the group will be a shadow of its former self.

But Mosul has significance beyond the fall of IS. Ever since Sennacherib made the city his capital in 700 BC, whoever ruled it has dominated the region—be they Assyrians, Babylonians, Arabs, Ottoman sultans or the British empire. It remains strategically important in the 21st century: regional powers regard a post-IS Mosul, if not as a jewel to conquer, at least as a place to deny to rivals. The Turks view it as a barrier to Iran’s expansion of influence westwards. Arabs fear what they suspect are the neo-Ottoman aspirations of Recep Tayyip Erdogan, Turkey’s president, to regain the city. Sparring Iraqi Arabs and Kurds consider Mosul crucial for control of northern Iraq. Meanwhile, Mosul represents a larger battle between America, which is directing the effort, and Russia. If it goes to plan, America will highlight the chasm between its operations in Iraq and Russia’s pummelling of the city of Aleppo, which in history, religious and ethnic mix and mercantile character is Mosul’s Syrian counterpart. If instead the American-led offensive results in a prolonged siege and the devastation of Mosul’s old city and its inhabitants, it will invite an unflattering comparison to the assault on its sister-city.

Much will depend on the city’s 1m Sunnis. Guaranteed a future in a post-IS Iraq, Mosul’s population might refrain from answering IS’s call to rise in the city’s defence. Elevated to control of the city, they could plug the vacuum outsiders might otherwise battle to fill. If they were embraced by the government in Baghdad, sceptical Sunni governments, particularly in the Gulf, might agree to participate in the reconstruction of Iraq’s battered Sunni provinces. Despite the appointment by Haider al-Abadi, Iraq's prime minister, of Najim al-Jabouri, a local Sunni general, to lead the campaign, many Sunnis fear that in Mosul as in Aleppo global powers are implicity helping Iranian-backed regimes ravage the last vestige of Sunni power in the Fertile Crescent. Allegations of massacres after Iraq’s reconquest of Fallujah in June did little to reassure. But if Shia militias are kept out of Mosul, and power devolved to the city’s inhabitants, Iraq might yet start to turn the page on its and the region’s sectarian wars.

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Even with backing from Kurds, Shia militias and an American-led international coalition, the campaign will be hard