More is riding on the battle for Mosul than the recapture of Isis' main stronghold in northern Iraq. Also on the line is the Obama Administration's theory that the extremists can be defeated in Iraq, Syria and elsewhere without American ground troops doing the fighting.
For more than two years, the US Administration has stuck to its argument that the only path to a sustained victory over Isis (Islamic State) group is for locals, not outsiders, to bear the main responsibility for the fighting and for governing after the extremists are removed.
President Barack Obama has taken a lot of political heat for that approach, which critics say has allowed Isis to expand its international reach and influence.
The viability of Obama's strategy has been widely doubted. In May 2015, after months of US bombings in Iraq and in the midst of Americans training and advising Iraqi ground troops, the Iraqis lost Ramadi. US Defence Secretary Ash Carter publicly said he doubted the Iraqis' will to fight. The US support role has grown and the Iraqi security forces have managed to retake key parts of western and northern Iraq, including Ramadi.
US airpower played a key role in the run-up to the fight for Mosul by taking out Isis defences, cash resources, supply routes and some of the group's leaders. The US is now providing air cover as Iraqi security forces and members of the Kurdish militia begin their attempt to retake the city. American advisers are working with Iraqi troops.
If Isis loses a crown jewel of its so-called caliphate, will that be a decisive and sustainable victory for Iraq? Or will Baghdad once again falter, allowing sectarian and political divisions to destabilise the country and permit a return of extremists?
Seth Jones, a defence expert at Rand Corp., says the combat phase of the battle will be "much easier" than the aftermath. "I think there's a strong possibility that a lot of the political grievances actually get accentuated."
David Petraeus, the former Army general, calls the Obama approach in Iraq and Syria "a new way of fighting. It's much more sustainable in terms of blood and treasure than obviously having our forces have to do it".
General Martin Dempsey, who was chairman of the Joint Chiefs of Staff when Isis moved into Iraq, said: "If we were to take control of this campaign ... then ... we would probably defeat Isis on, let's say, a faster timeline," but it would not last. "Maybe Isis goes away, maybe they're defeated militarily, and two years from now another group ... will just be back."
In the Obama view, Iraq is more likely to regain, and retain, control of its territory if it is not relying on US troops to do the fighting. Mosul is the biggest test of that theory.
700,000 could swamp shelter meant for 60,000
The military operation to wrest Mosul from Isis could potentially become the single largest, most complex humanitarian operation in the world in 2016, a UN official said.
Speaking via video-link from Iraq, Lise Grande, the UN humanitarian coordinator for Iraq, said that in the worst case scenario, one million civilians could flee the city with 700,000 of them requiring shelter - overwhelming emergency sites that currently only have the capacity to hold 60,000 people.
"Our capacity to support 700,000 people in the short-term - we couldn't do it. And certainly if we had to mount a response over the intermediate-term, if they couldn't go back to Mosul quickly, if there was too much damage in the city, then it would test us to the breaking point," Grande said.
She said that the UN was especially concerned about the safety of the estimated 1.2 to 1.5 million civilians inside Mosul who may get caught in the fighting. She said officials were also concerned that Isis had already booby trapped parts of Mosul and positioned snipers within the city.
"In the worst case scenario, we can't rule out the possibility that there may be a chemical weapons attack. We also fear that Isis, as they did in Fallujah, may try and hold civilian populations either as human shields or forcibly expel huge numbers of civilians in the face of an attack by the Iraqi security forces knowing the Iraqi forces will not fire on their own people."
Largely due to a lack of funding from the international community, the UN and its partners have only set up six emergency sites with a capacity to hold some 60,000 people. But Grande said in the next few weeks 22 emergency sites should be in place with a capacity to hold over 400,000 people - far short of the 700,000 people who could potentially flood out of the city.