IN A barren military camp near Mosul in Iraq, 500 balaclava-clad men train for urban warfare under the watchful eye of their leader, a former general in Saddam Hussein’s army. They are among 4,000 volunteers for the National Mobilisation Unit, a multi-ethnic force being assembled by Atheel al-Nujaifi, the governor of Mosul’s Nineveh province.

The men are preparing to be deployed to keep the peace in Mosul, if and when Iraq’s second city is retaken from Islamic State (IS). They would replace the police and military forces that melted away in the face of the jihadists’ onslaught last June. A combination of Iraqi soldiers, mainly Iranian-backed Shia militiamen and American air power recaptured Tikrit on March 31st. Now Mosul beckons.

Yet the fight there will be far harder. Mosul is a city of 2m people compared with some 300,000 in Tikrit, which took weeks to retake (there are still some IS pockets). Mosul is the Iraqi base of IS, which was present there long before it took the city.

It is still unclear who will try to recapture the place. Mr Nujaifi rejects any notion of a Shia-led intervention in the city. In Tikrit, Shia militias had to be pulled out after looting and revenge attacks on Sunni residents; last year hundreds of Shia soldiers were executed, and their bodies are now being exhumed. Haider al-Abadi, Iraq’s prime minister, said that the Hashid al-Shaabi, the umbrella-group for Shia militias, will not be sent to fight in Mosul.

Mistrust between Sunni Arabs and Iraq’s mainly Shia security forces runs especially deep in Mosul. Unlike other Iraqi cities that are more ethnically and religiously uniform, Mosul is home to myriad communities. Sunni Arabs, Shias, Kurds, Christians and Yazidis have all registered for Mr Nujaifi’s force. But Mosul also has a hard Sunni core, partly fuelled by disgruntled former officers under Saddam who bore the brunt of de-Baathification policy after America toppled Saddam in 2003. Iraqis call the place “the city of a million soldiers”. Some of them joined IS.

On April 6th the Kurdish president, Masoud Barzani, said his fighters, known as Peshmerga, would assist in the campaign to retake Mosul. But officials say they are likely only to provide support, not enter the city. So the task will most likely fall to the Iraqi army, which is still being rebuilt after last June’s debacle. “American command and co-ordination will be the key factor if Mosul is to be retaken,” says Michael Stephens of RUSI, a think-tank. Iran may not be happy about that. Mr Nujaifi says the government pays his men salaries of about $700 a month, but it has not sent them any arms. He uses his own money to buy guns on the black market. Officials in Baghdad cite a shortage of weapons, somewhat implausibly. Sunni officials, and some diplomats, say the delay may be due at least in part to the Shia leanings of the government. Many ministers mistrust the Sunnis, and the most extreme Shia militiamen label them all as IS. This all suggests that the battle for Mosul is unlikely to happen this spring. Lesser targets in Anbar province to the west of Tikrit may come first. Some Iraqis even suggest Mosul should just be left to IS.