It has taken two years of training a demoralised army, backed up by the air cover and special forces of the world's greatest powers, for Iraq to mount an offensive to recapture Mosul from Islamic State. Almost a week into the US-led onslaught, those running the campaign say they have identified what they think is a chink in the jihadists' armour.If local fighters in Mosul can be persuaded to drop their allegiance to Islamic State, there is a chance that the battle will last only weeks rather than months.Mosul, Iraq's second biggest city, is where IS leader Abu Bakr al-Baghdadi declared his Sunni caliphate in 2014.Now the battle to retake Mosul pits a 30000-strong Iraqi regular force backed by the US and European powers, alongside Kurdish and Shi'ite militias, against the jihadists.Islamic State fighters, estimated at between 4000 and 8000, have rigged the city with explosives, mined and booby-trapped roads, built oil-filled moats they can set alight, dug tunnels and trenches, and have shown every willingness to use some of Mosul's 1.5 million civilians as human shields.Islamic State seems to have a plentiful supply of suicide bombers, launching them in explosives-laden trucks against fighters converging on Mosul.Many of those involved in the battle for Mosul stress the need to break the allegiance Islamic State has won or coerced among alienated Sunnis.They believe many from Mosul itself may lay down their arms. Crucially, more than half of Islamic State's fighting strength comes from Sunnis initially relieved they were being freed from sectarian persecution at the hands of a Shi'ite-dominated government in Baghdad and a corrupt and brutal army.Some strategists believe those Sunnis could turn against the brutality of Islamic State rule - just as Sunni tribal fighters turned against al-Qaeda in Iraq a decade ago - if Baghdad guarantees their livelihoods.In the battle for Mosul, it has supposedly been agreed that neither Shi'ite fighters nor Kurdish Peshmerga will enter the city when it falls to avoid stoking a sectarian backlash.While the anti-Islamic State coalition has gained momentum, military strategists and intelligence officials say the closer the Iraqi forces get to Mosul, the harder it will be."If they decide to defend the city, then it will be more difficult and the process will slow down," the intelligence chief said.Once in Mosul, Iraqi special forces would have to go from street to street to clear booby traps set by Islamic State."The roads are very narrow. You can't use vehicles or tanks, so it will be a fight, person by person," said caliphate Karim Sinjari, Interior Minister in the self-governing Kurdistan Regional Government of northern Iraq.Until now, it has been easy for the coalition to hit Islamic State positions in deserted villages around Mosul, but the air strikes will slow down once Iraqi forces get into the city.Islamic State has succeeded in the past in blocking army troops from moving against it by staging suicide attacks and rigging explosives. But that would no longer be an obstacle in Mosul as the Iraqi army has recently received an effective guided missile system that destroys explosives-packed vehicles.The Iraqi commanders say their tactic now would be to cut Islamic State fighters off from the hinterland of supporting villages and then split the city into different neighbourhoods.Brigadier Haider Abdul Muhsin al-Darraji said military units would launch simultaneous attacks from multiple fronts on Mosul and divide the city into sectors to isolate Islamic State fighters. With the coalition launching air strikes, the jihadists will have little chance of getting reinforcements from the western side of the city, which has been left open to encourage their departure towards Syria.The difficulty is how to hit Islamic State targets inside Mosul without causing massive civilian casualties. "It's just like a tough surgery to remove a brain tumour," Darraji said.Colonel Mahdi Ameer said Islamic State had "deliberately blocked residents from leaving the city to use them as human shields and prolong the battle".The Mosul offensive will be the most important battle fought in Iraq since the US-led invasion in 2003. What happens next will shape or break an already fractured Iraq."There are growing concerns about fixing the political peace the day after liberating Mosul," said Hoshyar Zebari, a former finance minister of Iraq."How will this multi-ethnic, multi-sectarian city ... be governed and run without communal conflict, without revenge killing, without a large displacement of people?"But the battle against radical Islamists in the region would not end with the liberation of Mosul."Mosul will not be the end of Islamic State or the end of extremism in this region. They will go back to more asymmetric warfare. We will see suicide attacks inside Kurdistan, inside Iraqi cities, and elsewhere," said Zebari. - Reuters..