A long convoy winds down the mountain, the vehicles rumbling over a dirt track. The trucks and jeeps carry Iraqi Kurdish fighters clutching assault rifles as they head into battle. Straight ahead, a towering black plume rises from the ISIS-held city of Mosul, a smoke screen designed to block the vision of coalition warplanes bombing from the air.

This is the fourth day of the Iraqi-led offensive to reclaim Mosul, a city of about 1 million people and the jewel of the Islamic State’s self-proclaimed caliphate. It is the most significant battle in more than two years of international fighting against ISIS, which emerged after years of Sunni militancy in Iraq and chaos in Syria to seize huge portions of both countries in 2014 and displace al-Qaeda as the world’s most pressing terror threat. The Kurdish convoy is part of a three-pronged attack aimed at seizing a layer of villages east of Mosul, one of several maneuvers meant to set the stage for the main assault on the city, weeks or even months hence.

Leading one platoon is Tania Hassan, a 26-year-old lieutenant. As he awaits the order to advance, he steps out of his armored SUV and sits cross-legged on the ground, smoking a cigarette and twisting a piece of dry grass with his fingers. Hassan’s unit comes from one of the more professional branches of the Iraqi Kurdish militias known as peshmerga, or “those who face death.” His troops are trained by the U.S.-led coalition against ISIS. But he has a sober view of the battle ahead. “It won’t be easy. Everywhere is a trap,” he says. “Everywhere there are IEDs,” or improvised explosive devices, the bombs that killed thousands of U.S. troops during the occupation of Iraq that helped give rise to ISIS. “This is a main organ in ISIS’s body. Raqqa is the heart, but Mosul is no less important.”

Hassan climbs back in his SUV and the convoy lumbers forward once more before arriving on a ridge overlooking the battlefield, located in the village of Tiskharab Saghir, which sits on the eastern outskirts of Mosul. On the flat ground below, Kurdish fighters in humvees trade fire with ISIS fighters in the houses, the machine guns and rifles producing a steady crackle punctuated by the occasional roar of mortars and tank fire. After three hours of fighting, a vehicle rumbles out of the village: an ISIS suicide car bomber. Up on the ridge, members of Hassan’s unit erupt into a frenzy, running and shouting, Take it out! On the flat ground, Kurdish troops train a powerful machine gun on the car, detonating it with a blast. A couple of hours later, Hassan’s unit pushes across the field into the village. Although they gained a foothold, Hassan grimaces. The ISIS militants are digging in, and his unit is taking casualties. “We’re back to square one,” he says, clutching a walkie-talkie.

ISIS seized Mosul, Iraq’s second largest city, when the Sunni jihadist group swept across the country in a lightning offensive in June 2014. The Iraqi military, which the U.S. had spent years and billions of dollars trying to forge into an effective fighting force, melted away in the face of about 2,000 jihadists. And while the loss of Mosul was a military fiasco, it also demonstrated the potentially fatal political weakness of post-occupation Iraq. A series of Shi’ite-led governments in Baghdad alienated enough of Iraq’s Sunni Arab public that ISIS, despite its reputation for barbarity, managed to win a degree of support in largely Sunni cities like Mosul. Now, more than two years later, the fight to reclaim the city offers a critical test of the Iraqi state’s capacity to reclaim, rebuild and govern this fractious country as a whole, despite the regional and sectarian divisions that have plagued it since the fall of Saddam Hussein.

The military coalition now making slow progress toward Mosul reflects those divisions. Leading the battle is Iraq’s regular army, still smarting from its ignominious retreat from Mosul. It is joined by a motley of Kurdish militias that answer to the semiautonomous Kurdish Regional Government, based in the city of Erbil, just 50 miles east of Mosul, where the Kurds dream of independence. There are also Shi’ite-led militias that rose to prominence in the wake of the military’s collapse in 2014, creating a bulwark against ISIS but alarming non-Shi’ites who fear they will be the target of sectarian violence. The campaign as a whole is backed, tacitly or otherwise, by the U.S.-led international coalition against ISIS, which brings expertise, airpower and more than 5,000 U.S. troops on the ground.

Arrayed against the Iraqi forces are the cadres of the Islamic State, which is fighting to prevent the collapse of its once vast realm in Iraq and Syria. Over the past two years, the Iraqi military has driven ISIS out of its other key strongholds in Iraq, retaking the cities of Tikrit, Ramadi and Fallujah. In Syria, Kurdish militias and Turkish-backed rebels are seizing chunks of ISIS territory, on the way to an eventual offensive against the group’s de facto capital in the city of Raqqa. “The Iraqis are fighting with skill and commitment and courage, enabled by the coalition,” U.S. Defense Secretary Ash Carter said on Oct. 25, following a meeting in Paris with anti-ISIS allies after a quick tour of Iraq. “And today, we as members of the coalition resolved to follow through with that same sense of urgency and focus on enveloping and collapsing [ISIS’s] control over Raqqa as well.”

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But Mosul is special to ISIS–it was from the pulpit of the city’s Great Mosque that its commander, Abu Bakr al-Baghdadi, made his first sermon after the group declared the founding of a worldwide caliphate, in a moment captured in a video released on July 5, 2014. So ISIS will dig in and ISIS will fight. In areas retaken by the coalition during the early days of the offensive, Iraqi and Kurdish troops uncovered an elaborate network of tunnels that provided shelter from U.S. air raids and a hiding place from which to launch surprise attacks. Retreating ISIS fighters also left behind hundreds, if not thousands, of improvised bombs, rendering most of the once occupied villages unlivable even after liberation. In the desert south of Mosul, they set fire to oil fields and blew up a sulfur plant, sending poisonous clouds of thick smoke and acrid gas that blot out the sun and burn the lungs. Even though ISIS has withdrawn from some areas surrounding Mosul, it launched at least two counterattacks, one in the town of Rutba and another in Kirkuk.

The assault on Kirkuk, an oil-rich city of about 850,000, was particularly unnerving. On Oct. 21 about a hundred ISIS gunmen stealthily arrived wielding assault rifles, rocket-propelled grenades and explosive belts. They killed at least 99 civilians and security forces over two days of fighting. With dozens of gunmen and multiple targets, the assault in some ways mirrored the terrorist attack on Paris in November 2015, writ large. It also demonstrated that ISIS is far from finished as a fighting force. Even as it mounted the largest and most complex defense of a city in its short history–and after absorbing two years of U.S.-led airstrikes–ISIS still had the soldiers and resources to stage a major attack 100 miles southeast of Mosul, sowing chaos and fear among civilians far behind the front lines. The operation also appeared designed to exploit the tensions within the anti-ISIS alliance by striking a diverse city claimed by both Baghdad and the Kurdish government. The attack may further inflame sectarian tensions in a country where ordinary Sunni Arabs are often suspected of sympathizing with ISIS.

On the battlefield, ISIS has effectively slowed the offensive, making Iraqi troops pay for every inch of territory. On the fronts around Mosul, the jihadists have deployed snipers, improvised bombs and waves upon waves of suicide bombers. Their preferred variant of the suicide bomb is the so-called vehicle-born improvised explosive device–VBIED in military parlance, suicide car bomb to everyone else. On Oct. 20 alone, Iraqi special-operations forces fighting their way into the town of Bartella, on the main road from Erbil into Mosul, encountered 15 separate car or truck bombs. Two of the vehicles were destroyed by airstrikes, the other 13 by troops on the ground. They also encountered more than 50 IEDs likely laid by ISIS troops, while a single sniper wounded four men.

“We saw the ISIS snipers, and they’re not fleeing. They’re fighting. They’re resisting strongly,” says Colonel Falah Fadel Jasem, commander of the troops who seized Bartella. “They’ve been planning for this war.”

Jasem’s unit is part of Iraq’s Counter Terrorism Service, known as the Golden Division, a force that smashed its way through a series of ISIS strongholds earlier this year. The elite troops rolled into the Mosul area a day earlier in a convoy of humvees and massive mine-resistant vehicles. They established a temporary headquarters in Sheikh Amir, a tiny abandoned village on the Erbil-Mosul road, which was recently liberated by the peshmerga. When they arrived, the unit set up a command center and a field hospital in a pair of houses. The troops blared music, dancing on top of the vehicles and mugging for a television crew that turned up to document their arrival.

Most of those soldiers moved out within a matter of hours, pressing west into Bartella, a traditionally Christian community and the last town of any size before the outskirts of Mosul proper. The medics and support crew stayed behind in the bombed-out village, occupying an abandoned ISIS outpost. The jihadists had left behind signs of their presence, including an underground tunnel with two entrances, one in an open area near the main road, and one inside a house. Metal braces reinforced its walls and whole rooms were lined with thick white plastic–this tunnel had been the product of weeks of work. Inside the fighters had stashed canned goods and dozens of jugs of water. The underground shelter also appeared to have been used recently, with boxes of uneaten tomatoes and clothes strewn about the floor.

The ISIS fighters also left behind documents: Qurans and other religious books, newspapers containing updates on battles across the region. There was also a single page titled in Arabic, “Orders that must be carried out,” issued in February 2016 according to the Islamic calendar and signed by “Major General Jaffar al-Tayyar, the Emir.” The document gave the soldiers 12 bullet-pointed instructions: stockpile rations for a month, avoid gathering in the open, where they could draw the attention of airstrikes. In addition, “each station must have a solar cell for charging devices.” Some of the general’s instructions are bafflingly mundane. He orders the troops to have a generator and a stockpile of fuel on hand. He reminds them to pray.

The extent of the militant infrastructure in the village was an ominous indication for the battles to come. If ISIS is willing to put the time and care into digging such an elaborate tunnel in this tiny speck of a village, what is it planning in a major city like Mosul?

In the middle of the afternoon on Oct. 21, an ambulance barrels up the road to Sheikh Amir from the direction of Mosul, followed by a pickup truck. The ambulance doors burst open and medics carry out two wounded soldiers, one screaming in pain from burns on his face and limbs. They lay the severely wounded man on the tile floor outside the field hospital and start to bandage him. Another group of men, moving slower, carry a body wrapped in a blanket off the truck, setting it down on the ground nearby. They pull back the blanket to reveal a man in a brown jumpsuit, his left shoulder soaked with blood. The soldier had been driving a tank sent to cut the supply line from Mosul into Bartella, and ISIS attacked using an exploding humvee. A medical officer empties the dead man’s pockets: cigarettes, a phone, an ID card. Minutes later the men zip the body into a black bag. It is the platoon’s first death in the Mosul campaign.

Nearby the burned man is convulsing. “Please, can you tell me I’ll open my eyes again?” he wails. “My body is burning!” The field hospital is run by an energetic and slightly paunchy physician named Ahmed Hussein, 37, who speaks American-accented English from years of working alongside U.S. troops. Hussein tends to the wounded soldier. “My love, you’ll be O.K.,” he says in Arabic. They finish bandaging him and pack him back into the ambulance alongside the body in the bag. His wounds are too severe, the doctor says. He’ll be taken to a hospital at an American-run base.

A dour silence falls over the encampment. The Golden Division soldiers have already lost many of their comrades fighting ISIS elsewhere in Iraq, and each of these men knows they could be next. But soon Hussein is flipping through his phone again, showing me pictures of his daughters and cat. (Hussein’s daughters are in Baghdad, while the cat–which the doctor tried to bring with him into the field, along with 120 packs of cigarettes–had to be left behind in Camp Speicher, a major military base near Tikrit.) After an hour, a truck arrives carrying the evening meal: a greasy pile of ground meat, vegetables and fries served in Styrofoam boxes. The soldiers lay down mats over the blood smeared on the floor, the same floor where they treated their comrade. They sit and tuck in to the food. “We try not to get depressed,” says one soldier. “Of course we’re worried, but we can’t get depressed. We have to go on.”

As more soldiers come in from the fighting, the story of the battle starts to filter out. After spending the night on Oct. 21 in Bartella, Golden Division troops say they were attacked with mortars. One wounded soldier interviewed in the field hospital was convinced that the ISIS fighters were popping out of tunnels to ambush them. “We expected it would go much faster,” says Golden Division soldier Althear Mohamed Obaid, 31. “The information we got was that there were no fighters inside.”

Iraqi and Kurdish troops are doing most of the fighting and dying here, but the thousands of U.S. soldiers on the ground in Iraq are coming increasingly close to combat. Oct. 20 saw the first American service member lost in the battle for Mosul–U.S. Navy Chief Petty Officer Jason Finan, a bomb-disposal expert, who was killed by an IED explosion while retreating in an armored vehicle under ISIS attack. Finan was the fourth American killed in action in the two-year-old war on ISIS, and his death was another demonstration of just how deeply involved the U.S. remains in Iraq. “The closer they get to Mosul, the harder it will be,” said Army Major General Gary Volesky, the top U.S. ground commander in Iraq, speaking in a press briefing on Oct. 19. “But make no doubt, the Iraqi security forces have the momentum, and they know it. And so they are as motivated to get to Mosul as we are to help them get there.”

Few military experts doubt that Mosul will fall, sooner or later. When it does, Iraq will be faced with the even bigger challenge of picking up the pieces, which includes the hundreds of thousands of Iraqis forced out by ISIS, who are just now beginning to come home.

On Oct. 25, some of the first returnees arrived at a camp not far from Sheikh Amir, where the Golden Division outpost had been. The Iraqi military had retaken the village of Topzawa, on the eastern outskirts of Mosul, ordering the residents to leave while they cleared the town of explosives and any militants left in hiding. Groups of mainly women and children filed off the first buses, some of them into the arms of family members they had not seen for more than two years. They smiled with elation. They broke down and cried as they embraced cousins, uncles, spouses. And they looked fearful as peshmerga troops escorted them into a holding area surrounded by a chain-link fence.

They returnees described life under ISIS: dreariness–no smoking, no cell phones–punctuated by terror. But that, at least, was behind them. Noureddine Shamseddine Shahoud, 27, a man from the village who fled when ISIS took over, was in the camp to see his wife and children for the first time in years. “Life starts now,” he said.

–With reporting by MARK THOMPSON/WASHINGTON

This appears in the November 07, 2016 issue of TIME.

Contact us at editors@time.com.

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