In interviews with the ABC's Matt Brown, detained Islamic State fighters have detailed the mix of grievance and religious commitment that powers their movement.

As the US-led coalition and the Iraqi army turn their sights on Mosul, they are also planning for "the day after", to stabilise the city and heal the wounds caused by 13 years of invasion, insurgency, jihad and civil war.

Two fighters detained by Kurdish intelligence in northern Iraq have described their motivations for joining IS and offered telling insights into what it will take to truly defeat the group.

It is clear the extremists have been nourished by a deep, bitter well of anger, ideology and faith. But, as well as the international jihadists they like to publicise, their ranks are bolstered by simple, local men.

The first I met, Mohaned, 31, was captured on the front lines 50 kilometres south of Mosul, where he has a wife and four children.

His reason for fighting is simple.

"My motivation was due to religion," he said.

But Mohaned's motivations are actually more complex. His induction began when the US-led invasion toppled Saddam Hussein, ending his Sunni minority's dominance of Iraq, unleashing jihadists and a sectarian civil war. And, he has blood ties to the fight.

"When the Americans came and people started to wage jihad against them I learned about it little by little. I especially learned from my brothers," he said.

'My brothers … they were killed by the Americans'

US soldiers stand guard near suspected insurgents arrested during a raid in Baghdad in 2007. ( Thaier al-Sudani : Reuters )

Mohaned's family's connection to the jihadist insurgency was forged when the notorious Abu Mussab Al Zarqawi led the jihad after the American-led invasion, and maintained throughout the group's changing names and fortunes.

"The main reason I joined IS and not another group is that my brothers used to be part of this organisation," he said.

"My older brother was with Al Qaeda and the other one with the Islamic State of Iraq. They were killed by the Americans in 2004 and 2006."

Abu Mussab Al Zarqawi was a key figure in the Iraqi insurgency ( Reuters )

That is telling because after his last brother died, the American military surge brought relative calm to Iraq.

But, in Mosul, a formidable mix of insurgents and jihadists did not give up. And, by the time IS roared back onto the stage in 2014, it was Mohaned's turn to join the fight.

"When they kill Westerners, it's because Westerners were the ones who occupied Iraq. And God says if somebody encroaches on you, retaliate against them," he said.

In the intervening years, he said, abuse by Iraq's new, Shiite-dominated army alienated many in the mainly Sunni city.

"They took two people from our village and killed them later on," he said.

Mohaned left school after year 7. While he did not participate in the Islamic State group's religious indoctrination program, he absorbed the theocratic reasoning and slogans that justify their violence.

On the subject of civilian deaths, he cited a 14th century Islamic scholar favoured by jihadists.

"Ibn Taymiyya says if infidels hide behind Muslim prisoners in a confrontation then you can kill those Muslim prisoners just so you can kill the infidels," he said.

Given his family's long history of rebellion, it is perhaps no surprise Mohaned does not see the looming battle for Mosul as the end of the story.

"Even if [IS] loses Mosul and Raqqa, they will be present in other places. The caliphate will not be destroyed," he said.

That view was echoed by Ali, another Mosul local, caught 25 kilometres south-east of his hometown, where he used to run a photocopy shop to support his wife and child.

He said he did not join IS until they took over in 2014.

Ali was caught 25 kilometres south-east of Mosul. ( ABC News: Aaron Hollett )

Like Mohaned, Ali was nonchalant about the slaughter of non-combatants, and readily agreed I could be beheaded like other Western hostages.

"You know better than me. This [beheading] had to happen. This is a war between Muslims and crusaders. Weren't there beheadings in the past as well?" he said.

He said he, like many local Sunni men, joined IS after years abuse at the hands of the army.

"I was arrested three times. Many of them were Shias who harassed us and this was the major reason," he said.

And he argued it would be impossible for the Iraqi Government to wrest control of the city away from IS.

"Now many people believe in their ideology … even children. And I don't think that the Iraqi army's behaviour will get any better," he said.

If it is possible to renounce core articles of faith, cast off blood feuds, and put past abuses behind you, then it is also true that all three make for a potent mix.

And this snapshot into Mosul makes it clear, the Iraqi Government will have to be convincing on two key fronts: that it can destroy IS and keep them down — for more than a decade — and, in the meantime, offer a better, more secure life.

Before it will even get that chance, a bloody battle looms.