IRBIL • 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 the Islamic State in Iraq and Syria (ISIS).
Almost a week into the US-led onslaught, many of those running the campaign say the battle to retake the city could be long and hard.
But they have also identified what they think is a chink in the militants' armour. If local fighters in Mosul can be persuaded to drop their allegiance to ISIS, there is a chance that the battle can be brought to a more speedy conclusion, and that could have major implications for the future of Iraq.
The Mosul offensive will be the most important battle fought in Iraq since the US-led invasion in 2003.
Number of troops in Iraqi regular force backed by the US and Europeans, alongside Kurdish and Shi'ite militias
4k to 8k
Estimated number of ISIS fighters holding Mosul
Number of civilians in Mosul, who are suspected of being used as human shields by ISIS militants
Mosul, Iraq's second biggest city, is where ISIS leader Abu Bakr al-Baghdadi declared his Sunni caliphate in 2014, after his alliance between millenarian Islamists and veteran officers from the disbanded army of Saddam Hussein roared back into Iraq from bases they set up in the mayhem of Syria's war. Five Iraqi army divisions melted away before militants numbering merely in the hundreds.
Now the battle to retake Mosul pits an unwieldy coalition of a 30,000-strong Iraqi regular force backed by the US and Europeans, alongside Kurdish and Shi'ite militias, against militants who have exploited the Sunni community's sense of dispossession in Iraq and betrayal in Syria.
Not just its outcome, but the political sensitivity with which this battle is handled, could determine the future of ISIS and Sunni extremism, as well as the shape of this part of the Middle East, which is being shattered into sectarian fragments.
ISIS militants, estimated at 4,000 to 8,000, have rigged the city with explosives, booby-trapped roads, built oil-filled moats they can set alight, dug tunnels and trenches, and have shown every willingness to use Mosul's up to 1.5 million civilians as human shields.
ISIS would seem to have a plentiful supply of suicide bombers, launching them in scores of explosives-laden trucks against Kurdish peshmerga fighters converging on Mosul from the east and north-east, and Iraqi forces, spearheaded by counter-terrorism units, advancing from the south and south-west.
"Mosul will be a multi-month endeavour. This is going to take a long time," said a senior US official in Iraq.
Mr Karim Sinjari, Interior Minister in the self-governing Kurdistan Regional Government (KRG) of northern Iraq, said ISIS would put up a fierce fight because of Mosul's symbolic value as capital of its self-proclaimed Islamic caliphate. "If Mosul is finished, the caliphate they announced is finished. If they lose in Mosul, they will have no place, just Raqqa (in Syria)."
Adept at exploiting divisions among its enemies, last Friday's dawn assault by ISIS on Kirkuk, for example, was not just an attempt to divert Iraqi and Kurdish forces and relieve pressure on the main front.
It was also intended to galvanise Sunni Arab opinion against the Kurds, whose Iraqi peshmerga and Syrian Kurdish militia have fielded the most effective ground forces.
That is why many of those invested in the battle for Mosul stress the need to break the cohesion of ISIS and the allegiance it has won or coerced among alienated Sunnis in Mosul and beyond.
The opportunity is there, they say. They believe that while foreign militants will fight to the finish to protect their last stronghold in Iraq, the Iraqi fighters, many from Mosul itself, may lay down their arms.
"Most of the (ISIS) fighters now are local tribal fighters. They have some foreign fighters," said a senior Kurdish military intelligence chief. "If we can take this away from them, the liberation of Mosul is a job of a week or two weeks."
Fissures are widening inside the ISIS camp, with Iraqi, Kurdish and Western sources reporting resistance in Mosul and a spate of attacks on its leaders.
Mr Sinjari spoke of growing resentment against the group's brutality. "There is information that many people are revolting and carrying out attacks against ISIS. A number of ISIS members were killed on the streets at night."
This was confirmed by the US official, but could not be independently verified. It fits with accounts of a recent abortive uprising against ISIS, led by a former aide to Baghdadi, that ended with the execution of 58 militants turned dissidents.
Some strategists believe Sunni tribes could turn against the brutality of ISIS rule - just as the Sunni tribal fighters of the Sahwa or Awakening turned against Al-Qaeda in Iraq a decade ago - if Baghdad guarantees their lives and livelihoods.
In Mosul, there are Iraqi tribal people in ISIS who pledged allegiance when the group arrived, a Kurdish intelligence chief said.
"If the Iraqis send a message and reassure these Sunni Iraqis that they will be given a second chance, I think it is wise to do so, because if they put their weapons down, you are definitely taking out 60 per cent of their (ISIS) fighting force".
The official emphasised the need for the US-led coalition's close involvement in Mosul, especially after the experience of the recapture of Falluja, Ramadi and Tikrit - ISIS-held cities where refugees and local Sunnis suffered at the hands of Shi'ite militias.
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 a sectarian backlash.
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 Mr Hoshyar Zebari, a top Iraqi politician and former finance minister.
"How will this multi-ethnic, multi-sectarian city... be governed and run without communal conflict, without revenge killing, without a large displacement of people? That needs some political planning on how the city will be governed."