As the shock-and-awe of conventional warfare has given way to the unpredictable carnage of terrorism, much of the job of keeping America safe has fallen to the military's most elite soldiers. Delta Force. Green Berets. Navy SEALs. The special operators. But with new threats every day and the men stretched thin, can this new strategy last?

On his last flight home, three months before his death, Joshua Wheeler tore through a copy of D.H. Lawrence's Sons and Lovers. It was the only book he could find in English at a store where he was stationed, near Erbil, the capital of Iraqi Kurdistan. Wheeler was a master sergeant in the Army, a former Ranger, and a member of Delta Force since 2004. He was the recipient of eleven Bronze Star medals—four of them with the letter V, signifying valor in combat. This was his fourteenth deployment to either Iraq or Afghanistan. As he sat on the plane back to North Carolina, he read a hundred-year-old novel about a struggling London artist.

It was one of the things his wife, Ashley, loved most about Josh—his appetite for knowledge. "He would read whatever he could get his hands on," she says. They were married for two years, and in that time Josh had read more books than she could count. He especially loved history and anything related to international travel—anything that might help him know the world better. He kept a pocket dictionary in his car so he could look up new words at stoplights.

This was a rare leave for Wheeler. He was about a month into his four-month tour. But Ashley was about to give birth to their first child, a boy, and Josh was going home, if only for a short while. A few weeks before he left for Iraq, he and Ashley had moved into a new house in a patch of quiet woods near a small lake, not far from the base at Fort Bragg. "Our dream home," Ashley says.

Officially, the United States government does not acknowledge that Delta Force even exists. And yet in 2015, special operators from across the U.S. military were deployed to 147 countries, the most ever. They went to places where what we typically think of as warfare has devolved into a frightening morass of enemies who are both ruthless and scrappy, impossible to find and yet adept at social media. They went to places where large deployments of "boots on the ground" would not have made sense—and would not have been popular policy in the United States. They went everywhere. In fact, the United States special-operations forces—a handful of elite, highly trained, often clandestine units that make up only a tiny percentage of the total number of troops in the armed forces—were approaching burnout. "Post-9/11, it had become something of a mantra for our government: 'Just throw special forces against it,' " says Jim Reese, a former Delta operative who retired in 2007 after a twenty-five-year military career. "The operational tempo gets to a point where it's not tenable." Special ops were becoming the new way we fight in these places, whether the public knew it or not. There weren't supposed to be any more troops in combat in Iraq. That war ended in 2011. And yet here was Joshua Wheeler, just last summer, in the middle of his fourteenth deployment, and now going home to meet his boy.

U.S. Army Master Sergeant Joshua Wheeler

The leave lasted five weeks. Josh watched a lot of HGTV. He mowed the lawn and set up the backyard grill, which come deer season he would use to cook the meat he'd hunted. But he spent most of the time cooing over his newborn son, waking up early with him, holding him in his lap as he drank coffee. In a photograph that Ashley keeps with other favorites in an album titled "Our Father and Hero," Josh is napping on the couch, peaceful and happy, the baby tucked securely and sound asleep under his arm.

One of the last afternoons, he and Ashley took the baby for a drive. There was a burgeoning wine country in the part of North Carolina where they lived, and Josh and Ashley liked to stop in and taste the wine at as many wineries as they could. Josh pulled the car into one they hadn't tried before. It was a beautiful, warm day—perfect for sipping merlot. They sat on a hill overlooking the vines and an old barn. The boy lay in the grass in front of them.

Ashley looked at her husband. "Is it always going to be this wonderful? This perfect?"

Josh smiled.

"Yes," he said.

"We were already dead. Then God sent us a force from the sky."

On September 20, 2001, nine days after four airplanes had destroyed the Twin Towers, parts of the Pentagon, and thousands of lives, President George W. Bush stood before the entire U.S. Congress—before the entire world—and told of a new and imminent fight against terrorists and the governments that support them. "Our response involves far more than instant retaliation and isolated strikes," the president said. "Americans should not expect one battle but a lengthy campaign unlike any other we have ever seen. It may include dramatic strikes visible on TV and covert operations secret even in success."

His forecast, that fighting would increasingly be done in secret, proved especially true, and continued into the Obama administration. Before 9/11, there were roughly thirty-three thousand members of U.S. special-operations forces. Today, there are approximately seventy thousand. The budget for special operations has tripled since 9/11, to around $10 billion. Meanwhile, the military as a whole has been shrinking. In 2015, active Army personnel dropped below five hundred thousand for the first time in ten years and is projected to return to lower, pre-9/11 levels by 2019.

As special-operations forces have grown, so has their visibility. There was the daring Navy SEAL rescue of the crew of the Maersk Alabama cargo ship held hostage by Somali pirates in 2009. The dramatic 2011 SEAL Team Six raid that killed Osama bin Laden in his hideaway compound in Pakistan. (Before the raid, the existence of Team Six, now known as the Naval Special Warfare Development Group, or DevGru, was classified.) In Syria, the May 2015 attempt by Delta Force to capture Islamic State financier Abu Sayyaf that resulted in his death. The Alabama rescue was made into a movie, Captain Phillips, starring Tom Hanks, which grossed more than $200 million worldwide. The Bin Laden mission, too, went to Hollywood, as Zero Dark Thirty, nominated for an Academy Award for best picture. And multiple members of SEAL Team Six wrote memoirs of their experience. Special-operations forces, long the mysterious and unpublicized divisions of the world's largest military, had been deployed so prolifically and on such important missions that they had gone mainstream.

But these conventional, or direct-action, strikes—kicking in doors and taking out the bad guy—make up only half of special operations. The other is something more delicate, closer to spycraft. It's called unconventional warfare, and it involves working with a local population to foment insurrection against an undesirable government or terrorist group. Infiltration. Propaganda. False flag attacks. Guerrilla combat. You won't watch a movie about it—in fact, if all goes well, you will never know it even happened.

As the neatly defined wars of Iraq and Afghanistan wound down, this kind of shadowy conflict—hasty, dangerous, executed in the dark of night by small groups in parts of the world where the laws of war are murky—had become the rule rather than the exception. The experts even had a name for it: the gray zone.

They usedto be called Jedburghs.

On a warm night high above the coast of Brittany, France, the bomb bay opened on a B-24 Liberator and three men tumbled out. Their parachutes opened. They landed gently in a secluded area. Quickly, they were greeted by a small party of French resistance fighters, alerted to their arrival by coded messages embedded in a broadcast on the BBC. These were the first special-ops guys.

Most military historians date the origin of U.S. special-operations forces to Operation Eagle Claw, the military's failed 1980 attempt to rescue the hostages being held in Iran. But a version of them existed in World War II. The United Kingdom created an entity known as the Special Operations Executive, part of whose job was to blend into the French population and cultivate a resistance against German occupiers. They called on America for support, and starting in July 1944, U.S. commandos called Jedburghs parachuted into French territory. They worked in three-man teams, helping coordinate attacks against the Germans and keeping the various French resistance factions from fighting one another.

"Everyone in Special Operations Command knew about the Jeds," says retired Lieutenant General Charles T. Cleveland, who served from 2012 to 2015 as the commander of U.S. Army Special Operations. "They were a big part of the lore."

As commander, Cleveland oversaw the Army's entire special-operations forces. Years before, as a young officer, he had battled drug lords in Bolivia and planned sabotage missions in the Soviet Union. He had seen it all—Charles T. Cleveland was the man, the epitome of special forces—and he was proud of the recent high-profile missions: Maersk. Bin Laden. Sayyaf. But now he was concerned.

"Everyone in Special Operations Command knew about the Jeds"

In the early days of the Afghanistan conflict, Green Berets rode into Kabul alongside the horse-backed warriors of the Northern Alliance. Special-forces operators did the same in northern Iraq with the Kurdish army, known as the peshmerga. Then the wars bogged down in insurgency and nation building, and many of the tasks once reserved for top-level operators like Delta and Team Six—the assimilation, the mixing in with society, the searching for both allies and foes in plain sight—were handed off to the Army Rangers, and sometimes even to basic infantry. But Cleveland now worried—worried a lot—that direct action had come to dominate the strategy, and that unconventional warfare was becoming a lost art.

"We had a hundred days of brilliance in Iraq and Afghanistan, followed by four thousand days of strategic muddle," he says. "We weren't seeing the conflict properly. We thought traditional air and land battles of attrition could win the day. In reality, those tools were becoming less and less useful. It required a new way of thinking."

In the spring of 2013, Congress mandated across-the-board spending reductions—what was known as sequestration. This meant a potentially drastic reshuffling of the military's makeup. But where most military officials saw chaos, Cleveland saw opportunity: to resurrect the Jedburghs. Just over a year later, in the fall of 2014, the Army quietly opened a new division at Fort Bragg known as 1st Special Forces Command, which would ally five active-duty and two National Guard special-ops groups—about sixteen thousand troops total. Within the division would be small three-man teams of Green Berets, specially trained in both the art of combat and the science of social movements and terrorism. Between missions, they would spend years in the classroom studying languages, political theory, and the history and culture of specific regions. And they would be called the Jedburghs.

Lt. Gen. Charles T. Cleveland (right) with Ash Carter (left) at Fort Bragg in 2012.

The strategy shift that Cleveland championed has become even more important as the threats facing the nation have grown increasingly unpredictable, unorthodox, and terrifying. ISIS is a foe unlike any other—nationless, leaderless, recruiting participants from around the world, staging and broadcasting beheadings, killing thousands of civilians. Vladimir Putin's Russia first forcibly annexed Crimea and is now trying to do the same in other eastern regions of Ukraine, led by a proxy army of non-uniformed "little green men." Borders or treaties have been rendered obsolete. This is the gray zone.

In January, in the military academic journal Joint Force Quarterly, Cleveland published a paper titled "Unconventional Warfare in the Gray Zone." One of his coauthors was four-star Army General Joseph L. Votel, the current head of U.S. Special Operations Command who was recently nominated by President Obama to lead U.S. Central Command. Their goal was to define combat in this new and unpredictable world. "It's the sort of conflict that is below the threshold of traditional war," says Cleveland. And because of a reluctance to commit forces, the need for special operators has skyrocketed.

Hence the fourteen tours for men like Joshua Wheeler, and the warnings of burnout by men like Jim Reese, and the uncertainty that hangs over the nation's ability to sustain this kind of combat. And the joke special operators always make when someone mentions "boots on the ground"— their uniforms should include sneakers.

In early 2015, the Army put out a call for five thousand new special-ops candidates. And the demand is only expected to rise in the coming years, due to everything from the continued spread of ISIS to melting ice in the Arctic, which will create tensions over newly accessible territory. (The Army's Northern Warfare Training Center in Black Rapids, Alaska, trains special operators in skiing, snowshoeing, and other cold-weather skills.) But finding more special operators is neither quick nor easy. For instance, only 37 percent of applicants are accepted for the Army's Special Forces Qualification Course, which typically lasts more than a year.

"What it really takes to get through training isn't just athleticism or even physical toughness," says Jeff Eggers, a retired Navy SEAL officer and former special assistant to President Obama for National Security Affairs who now serves as a senior fellow for the nonpartisan think tank New America. "It is a high degree of mental toughness, the ability to work with a team, and the intellectual adaptability to approach problems from a variety of unorthodox ways. That's the rare mix of qualities."

"It's the old Apache theory: Run that horse till it drops, then eat it."

Such problem solving and maturity can't always be taught in the classroom or in war games. Rather, it comes from real-life experience. "The average age of a Delta Force member is around thirty-five," says Reese. "We're not recruiting high school quarterbacks. Delta is known as the Eagles, because eagles are not a flocking bird. They don't follow the pack."

To accommodate the demand, then, the pace of their deployments has quadrupled over the past decade. The ratio of time at home versus time deployed is close to one-to-one, though military officials hope to eventually get it to two-to-one.

"People can't stay productive," says Reese. "It's the old Apache theory: Run that horse till it drops, then eat it. You get guys who came in for a career in special ops, but after five or six years, they're worn ragged."

Like many special operators, Joshua Wheeler didn't talk about the stresses of work with his wife—didn't talk about his work at all. And Ashley didn't ask. "My father spent twenty years in a plant and I never really knew what he did either," she says. But Josh was starting to talk about life after the service.

Wheeler with his newborn son in 2015, shortly before his death in Iraq.

He was thirty-nine. He had enlisted when he was nineteen as a way to escape poverty in rural Oklahoma. His father died when he was young, and he was raised mostly by his grandparents. In turn, he helped take care of his brother and four half-sisters—changing diapers, getting them off to school, and making sure the fridge always had something in it, which sometimes meant hunting deer. Even with Ashley, he always insisted on keeping deer meat stocked in the freezer, his little way of ensuring she would never go without. "He was one of those very rare people who are able to claw their way out and just completely change their lives," she says.

Now, after two decades in the military and with a baby at home, Josh was ready for another change. He wanted to devote more time to his family. (His three sons from a previous marriage lived nearby.) He talked of getting into real estate, or maybe becoming a history teacher. He would have been great.

"He loved the Army," Ashley says. "But I really felt this time he didn't want to leave home. It was the first time I felt that."

The five Chinook and Black Hawk helicopters swept low across the northern Iraq desert. It was a little after two in the morning on October 22, 2015, and the sky was black and still. On board were forty-eight counterterrorism commandos from Iraqi Kurdistan and twenty-seven American elite operatives, including Joshua Wheeler. The rescue mission was both a direct-action strike and a perfect example of unconventional warfare—the first time the U.S. commandos had teamed up with the Kurds for a combat operation since the Iraq War began in 2003. The Americans weren't in charge. They were just there to advise and to assist only if absolutely necessary.

The call had come in only a few hours earlier. American surveillance drones had spotted figures outside a suspected Islamic State prison compound, near the Iraqi town of Hawija. The figures seemed to be digging trenches. Intelligence analysts believed there would be a mass execution in the morning.

Helmet cam footage of the 2015 raid in which Wheeler died.

The seventy prisoners—Iraqi soldiers and policemen, civilians from nearby towns—had already experienced months of brutal beatings and mock executions. They had seen cell mates taken away for interrogations, only to return broken and bloodied—or not at all. Rudaw, a Kurdish news outlet, interviewed some of the prisoners later. "They tortured us with electricity and put bags on our heads until we could not breathe," said one, Mohamed Hassan Abdulla, a police official from Alkhan, a village in Kirkuk.

As the helicopters closed in, American fighter jets "prepared the battlefield," dropping bombs on nearby roads and bridges, cutting off the path for any ISIS reinforcements. Then the commandos surged in, assault rifles ready.

According to a New York Times account, which anonymously cited a former Delta Force officer briefed on the mission, the plan was to detonate several holes in the compound walls and get inside. But a few of the Kurdish soldiers had trouble setting one of the explosives properly. Wheeler rushed over to help—this is what the Americans were here for. To guide, to assist, to step in when needed. They blasted the hole, and Wheeler ran through it. On the other side, he was met with a spray of enemy bullets.

All of the prisoners were freed. "We were already dead," one would later say. "Then God sent us a force from the sky."

As many as twenty ISIS fighters were killed. The only casualty among the American and Kurdish forces was Joshua Wheeler.

He was the first American to die in combat in Iraq in nearly four years—the mission would likely have remained secret if not for that newsworthy fact. The next day, at a press conference in Washington, D.C., Secretary of Defense Ash Carter faced a frenzy of questions from confused reporters. Why were American troops still in Iraq? Wasn't the war over? Hadn't the White House repeatedly insisted there were no boots on the ground?

All perfectly reasonable questions in another, earlier era of combat. But not now. Not in the gray zone.

"There will be more raids," Carter said bluntly. "They will be in harm's way. There's no question about it."

He was more eloquent about Wheeler's sacrifice: "This is what is so consistent and so amazing about the American soldier: He ran to the sound of the guns … I'm immensely proud of this young man. But pride doesn't make it any easier to welcome him home, fallen."

Josh had only eleven more days left in his tour.

Months later, Ashley still sometimes feels like she is in shock. He did everything in life so well, so thoroughly and competently. "I never worried," she says. "I never thought in a million years that anything would happen to him."

The dream home in the woods is still much the same way Josh left it before returning to Iraq. The grill is still out back. The fridge is stocked like he would want it. But some things have changed. Ashley has disconnected the doorbell so she won't have to hear it ring, like it did that night when military personnel showed up at her door, to tell her.

And in a place of honor in the living room, she keeps the copy of Sons and Lovers. Inside the front cover is a brief inscription Josh wrote to his family on that last flight home.

Good book. I hope someone reads it.

And to his unborn child, a simple declaration—one that suggested there would be plenty of time to say more.

I am flying over the ocean right now to see your mother give birth to you.