In Iraq, the U.S. is coaching the nation’s rival religious sects and ethnic factions to join forces for the biggest offensive on ISIS yet. Photograph Courtesy Corporal Andre Dakis / U.S. Marine Corps

America’s front line facing the Islamic State is more than two thousand miles from Brussels, as the crow flies, and then another ninety minutes by country road from the Kurdish capital of Erbil, in northern Iraq. The trip to Camp Swift, in Makhmour, the forward U.S. base, can be deceptively pastoral. I was slowed by a flock of sheep and goats crossing the road to a grassy plain sprinkled with budding yellow wildflowers. A curly-haired eighteen-year-old sheepherder, Mustafa Maghdid, picked up a young lamb to show me. A woolly white ram played at his feet. Millions of Iraqis fled as ISIS blitzed through the north, in 2014, but a determined few have been reluctant to surrender their herds or small farms. Tales of ISIS’s plunder are rampant. There is little left, according to the war grapevine, for those who may one day want to return.

The farming district of Makhmour is also one of the areas where ISIS has used primitive but deadly forms of chemical weapons—mustard gas and chlorine—since last August, most recently last month. It’s also the place where a Marine was killed this month by ISIS rocket fire. He was the second American killed since the U.S. withdrawal from Iraq, in 2011.

The number of American troops in the campaign against ISIS—Operation Inherent Resolve—has quietly escalated in recent months, to somewhere between four thousand and five thousand. (Significant numbers of personnel on temporary duty aren’t included in the formal headcount.) The Pentagon was in the awkward position of announcing the death of Staff Sergeant Louis Cardin, on March 19th, before disclosing that his group of two hundred Marines, from the 26th Marine Expeditionary Unit, had been deployed to Iraq a month earlier. Eight others were injured, three critically. ISIS knew about the new Marine station at Fire Base Bell before the American public did. The Marines were deployed to provide force protection—basically, artillery cover—for Camp Swift. In a familiar pattern, they were assigned a mission that the Iraqi Army is unable to do.

For all that is at stake, Camp Swift is a rudimentary facility, marked by a small, hand-painted sign, secluded inside a rustic military base run by the Kurdish Peshmerga. These fighters, many armed only with vintage Russian rifles, blocked the ISIS advance into Kurdistan in 2014 after tens of thousands in the Iraqi Army—trained and equipped with sophisticated U.S. tanks and artillery, at a cost of billions of dollars—fled, abandoning equipment and shedding their uniforms. In late 2015, the Peshmerga quietly opened their base in Makhmour, which is fifteen minutes from the front line, so that the Americans could set up their own shop.

I passed through the Peshmerga sentries and concrete blast walls that ring their base just as a toylike drone buzzed noisily on liftoff from inside Camp Swift. Warplanes from the U.S.-led coalition thundered above cloud cover. U.S. drones identify targets—ISIS fighters, arms caches, installations, tunnels, vehicles—that coalition warplanes then strike. The pace has picked up significantly in recent weeks as the campaign against ISIS, also known as Daesh, has escalated. The warplanes often hit more than a dozen targets in a day.

Inside the base, Peshmerga fighters were lolling outdoors on stoops or rickety chairs, in small groups, many smoking thin white cigarettes. “Daesh attacks mostly at night,” Zeryan Sheikh Wasani, the deputy local commander, told me. “During the day, air strikes can find and pick them off.” With its big windows and beige exterior, the Peshmerga headquarters looked more like a public school or apartment block; the building’s front door was unlocked. A row of armored Humvees—several with bullet-pocked windshields and machine guns on top—were parked in the gravel courtyard. The Humvees were provided to the Iraqi Army by the United States and then seized by ISIS after the Iraqis abandoned their transport in 2014. The Peshmerga later captured them in clashes with ISIS. The Kurds finally got their own American equipment—by fighting ISIS for it. “It’s our best matériel,” Wasani told me bitterly.

Camp Swift sits behind its own fortified walls, which are topped with big curls of razor-wire. The entrances have thick metal doors; small cameras monitor the periphery, including the Peshmerga. Inside is a maze of boxy little rooms—no windows—constructed of black-coated plywood. Colonel Scott M. Naumann is the commander of the 1st Brigade Combat Team in the 10th Mountain Division, based at Fort Drum, New York. He is the senior American in Makhmour. As I entered a tiny briefing room, he said, “If we take indirect fire, go out the door, turn right, then left, and head for the bunker.”

At Camp Swift, the United States is coaching, coördinating, and cajoling Iraq’s rival religious sects and ethnic factions to join forces for what will be the biggest offensive yet in the war on ISIS—the campaign to liberate Mosul, some sixty or so miles northwest. Makhmour, along with a second base, in nearby Gwer, is expected to be a key launching pad.

Mosul and Raqqa, in northern Syria, are the strategic centers of the Islamic State’s primordial caliphate. Mosul, with a prewar population of more than two million, is Iraq’s second-largest city and a former commercial hub. It will be critical to the political future and coherence of Iraq. The United States estimates that ISIS has deployed some ten thousand fighters—nearly half of its jihadi volunteers from dozens of countries—around Mosul. Raqqa is the Islamic State’s proclaimed capital, but it has only a fraction of the population, infrastructure, and resources of Mosul. Recapturing Mosul would be a huge psychological and military victory.

Officially, the Americans involved in Operation Inherent Resolve defer to Iraq in the campaign against ISIS. “It’s kind of like being the coach of a sports team,” Naumann told me. “We provide the best equipment that we can, the best advice we can on tactics and strategy, and on how to approach problems. When it’s time for the competition to take place, we have to stand on the sidelines. This is an Iraqi fight. It’s not an American fight.

“They do it on their timeline, too,” Naumann added. “We enable them, with drones, airstrikes, tactical planning. We offer a different perspective. We put those things together—and then they do their thing.” Naumann walked me through the modest forward-operations center, nicknamed the Bull Pen, where rows of U.S. troops (I wasn’t allowed to cite how many men, or even how many rows) monitor big screens and computers that track, live, what the drones see and what the airstrikes do. Unlike the current air campaign in Syria—and unlike the earlier intervention in Iraq, which ended in 2011—this time the United States has to get approval from the Iraqis to hit proposed targets. On more than one occasion, the Iraqis have said no.

The U.S. mission includes training and reëquipping the Iraqis, providing intelligence and airpower, advising and coördinating strategy, and, crucially, keeping the Iraqis united and focussed on Mosul rather than on each other. “We all know that if they do this on their own, it will be more longer-lasting . . . win for the future of Iraq,” Major General Richard Clarke, the commander of coalition land forces in Operation Inherent Resolve, told reporters last month.