Erbil, Iraq (CNN) In the distance, through the dusty haze that envelopes the arid plains of northern Iraq, we can just about make out ISIS on the horizon -- readying for a fight.

In one of the lifeless villages along the main road to Mosul -- a city once inhabited by more than 2 million people before it became the main prize in the Iraqi portion of the militant group's self-declared caliphate -- a dozen or so tiny figures scatter around on bikes and in trucks.

These villages are deserted, we're told, aware that an ugly fight is coming farther down the highway, and that Mosul represents the last ISIS bid to dig in. Pancaked buildings and a large Ferris wheel -- signs of the life there that was, has been extinguished, and may take years to come back.

As to when this fight begins is an open question. Meanwhile, political clamoring grows: Turkey, Iranian-backed militias, the Iraqi government -- all insistent that their red lines must not be crossed. But on the ground, a sense of momentum is building with the Iraqi and Iraqi Kurdish Peshmerga forces we meet.

The deserted villages of Bazgirtan and Ba-Sakhra, the last two villages before Mosul

ISIS dug in

The assault is imminent, and it appears ISIS inside Mosul knows that.

Trenches have been dug around the eastern side of the city -- an area less of an urban maze than the west side, into which many ISIS fighters have reportedly retreated. These have been filled with oil, to burn and cause a choking and blinding smoke during the attack -- to obscure their positions from coalition strikes.

Concrete walls are in place. Fighters are again wearing masks at checkpoints, to disguise their identities like they first did when the took over the city in 2014 -- a sign their confidence is decreasing and that they're worried about retaliation from Mosul residents.

In a bid to penetrate the most sacred privacy of an Iraqi home, women are being used by ISIS to search houses, even check other women to see if they're hiding precious SIM cards. Communication with the outside world is something ISIS has struggled to stop.

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Signs of strain

It's all the more surprising then, that one man from inside the city is prepared to head to its outskirts to get a phone signal and tell us about the resistance network against ISIS that he's part of.

We don't give his name, and he refuses to provide certain details -- such as the ISIS targets they'll hit when the broad assault against the city begins, and what the signal to launch that attack will be.

This is a small group of resistance fighters, whose cells know little of the other cells, to limit the ability of ISIS to penetrate them.

They (ISIS fighters) harass men if they don't have beards ISIS resistance fighter

The man says ISIS fighters are showing signs of strain. "There are more checkpoints, they harass people. They are making people change the registration plates on their cars from regular Iraqi government-issued plates to ones that say Islamic State. And they harass men if they don't have beards," he explains.

The focus now has become on-the-spot fines for transgressions of their strict Islamic code -- an effort to get money from locals quickly.

Another man who also says he's part of the resistance helping to coordinate from outside the city, told us his group had about 30 to 40 weapons -- mostly pistols taken from an Iraqi police unit when ISIS first overran the city in 2014.

Sometimes, after they kill an ISIS member, the pistols are immediately thrown away to prevent resistance fighters from being caught with the murder weapon.

"ISIS members are everywhere; they are on the streets, in the markets, in the cafeterias, in the restaurants. But we choose important people [to attack]," he tells us.

"When ISIS loses a member -- a foot soldier -- they don't care about that. Those people are paid daily or monthly for a certain service. We choose their influential and effective members with prominent positions in their organization."

Old Soviet-era mortars at a Peshmerga forward base on a mountain overlooking Mosul

M for Resistance

While one coalition official we spoke to said the liberation plan is not dependent on a large uprising inside the city to boost its success, the resistance insists it will play a seminal role in securing popular support for the forces that attack ISIS front lines around Mosul.

The increasing confidence of anti-ISIS activists is clear in the rare footage that CNN obtained from the city streets.

A man is shown driving along Mosul's mostly empty main roads, holding up a piece of paper beneath the phone's camera, showing the date and the letter M, in Arabic. This stands for Muqwama -- or resistance -- and the act itself could get the activist killed if caught.

But the videos continue to emerge, one even showing a burned-out car they claim was torched by them and belonged to ISIS.

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Exactly how the assault begins weighs, too, on the minds of those locals who will resist ISIS.

Ranged against the city are Iranian-backed Shia militia, who some locals fear may infiltrate the Iraqi army -- the force supposed to be behind the main assault.

Worries are spreading this group may commit atrocities against the local Sunni population -- despite their denial that they intend to do so. To add to the heady rhetoric ahead of what may be the worst and most complicated offensive yet against ISIS, Turkey has thrown its weight behind the Sunni forces that intend to assist in the assault, and try to calm the sectarian fears of the city's mainly Sunni population.

If the Iranian-backed Shia militia enter the city, Ankara says it may intervene militarily. The agreed "plan" is for that not to happen. But few have confidence in all sides to stick to the plan.

As one leading Sunni official in northern Iraq told me, "there is no Plan B." What follows will likely be uncharted chaos.