November 2004, against a shattered wall in south Fallujah in Iraq, with video rolling, I conduct a battlefield interview with US Marine Corporal William Wold. He has just shot six men dead inside a room adjoining a mosque and is juiced with a mix of adrenaline and relief.

He describes the 30-second sequence with a profane candor I have never seen matched in my decade of reporting on war around the world. ‘It was a fucking small room, dude. It was fucking small!’ He shakes his head. ‘Thirty-five fucking rounds. I was fucking scared dude. I fucking grabbed my nuts.’ Then, with one hand, he does so again, and lets out a big ‘Ohhh!’

‘I was told to go the room,’ he says, ‘and my first Marine went in… he saw a guy with an AK, I told him to shoot the guy, then I shot the six guys on the left… and my other Marine shot two other guys.’

Wold grew up near Vancouver in Washington State. A high-school linebacker, he had a college football scholarship waiting for him, but gave it up to join the Marines. His first assignment out of boot camp was with a small unit assigned to protect President George W Bush.

Now, here in Fallujah, the site of what will become the most famous battle of the US war in Iraq, the 21-year-old is covered in sweat, dirt and grime, which does nothing to diminish his charisma and good looks. We talk through the sound of machine-gun fire, tanks and even an air strike, the explosions providing unnecessary emphasis to his remarks.

‘My fiancée’s worried that I’m not going to come back the same. I’ll never tell her what things I did here. I’ll never tell anybody. ’Cause I’m not proud of killing people. I’m just proud to serve my country. I hate being here but I love it at the same time.’

Wold’s fiancée was right. He wouldn’t come back the same. He thought his war was over, but a few months later, back in the safety of his childhood home surrounded by his adoring family, the dark secrets and all the guilt emerged from his mind – like the Greeks from their hollow wooden horse, unrelenting in their destruction of ancient Troy.

The story of the Trojan horse, delivered as a gift but transporting lethal agents instead, has long served as an allegory for the destructive power of secrets – like the unaddressed guilt hidden in the minds of soldiers, repeated with every homecoming for thousands of years. War’s simple premise, killing, is like that Trojan horse, devastating those sent to do it and, ultimately, the society they return to when the war is done. The insidious damage is only made worse because wartime killing, a philosophically problematic act, has been left out of the global dialogue. After all, how can humanity’s greatest civil crime, killing, become heroic in the context of war? There are practical considerations as well: will too much discussion of killing make soldiers hesitate or even rebel against protecting us from threats?

I recognized the dissonance after completing a project for Yahoo News in 2006 called In the Hot Zone, in which I covered every major war in the world in one year. In 368 consecutive days of travel, 71 airplanes, 30 countries and 21 wars, the indisputable truth I found was this: combat is almost always the shortest and smallest part of any conflict, while collateral damage or civil destruction is war’s most enduring legacy. But even more surprising to me was that former combatants often become casualties themselves. War veterans I met across the globe, from Somalia to Sri Lanka, feel that they killed a part of their own humanity every time they pulled the trigger, becoming collateral damage as well.

Karl Marlantes, a former US Marine lieutenant in Vietnam in the late 1960s, says he and his fellow soldiers lacked context for the killing they would have to do. ‘When I did eventually face death – the death of those I killed and those killed around me,’ he wrote in his book What It Is Like to Go to War (2011), ‘I had no framework or guidance to help me put combat’s terror, exhilaration, horror, guilt and pain into some larger framework that would’ve have helped me find meaning in them later.’

What we’re beginning to learn now is that, of all those things Marlantes mentioned, unaddressed guilt might be the most dangerous for returning veterans. A recent study by the US Department of Veterans Affairs (VA) shows that nearly two-dozen veterans are killing themselves every day, nearly one an hour. This attrition, connected at least in part to combat-related post-traumatic stress disorder (PTSD) and other war-related psychological injuries, is an enormous price to pay for avoiding the subject. So great, in fact, that the total number of US active duty suicides in 2012 (349) was higher than the number of combat-related deaths (295).

If soldiers felt nothing about taking the life of another human being, that would be indicative of sociopathy

VA researchers recognised the epidemic, and over the past five years conducted a series of studies trying to drill down. Overwhelmingly, the work showed that veterans who killed others in war were at greater risk of psychiatric problems and psychic break. In a 2010 paper in the Journal of Traumatic Stress, VA researchers studied 2,797 US soldiers returning from Operation Iraqi Freedom. Some 40 per cent of them reported killing or being responsible for killing during their deployment. Even after controlling for combat exposure, killing was a significant predictor of PTSD, alcohol abuse, anger, relationship problems – and suicide risk.

Armed with these results, VA clinicians developed a disruptive new theory they’ve termed ‘moral injury’ – the notion that it’s not simply witnessing trauma that undoes combat veterans, but guilt; and in particular, guilt over two things: killing and not being killed. The implication is that we humans are fairly resilient in our ability to see horrible things and somehow continue functioning, but we’re not so good at living with what we consider our more shameful deeds. Even if killing seems justified by the demands and duties of war, it sends our moral compasses spinning.

According to the VA psychologists Shira Maguen of San Francisco and Brett Litz of Boston, both experts on military trauma, the key precondition for moral injury, our so-called Achilles’ heel, is a sense of ‘transgression’, a betrayal of what’s right. ‘In the context of war,’ they write, ‘moral injuries may stem from direct participation in acts of combat, such as killing or harming others, or indirect acts, such as witnessing death or dying, failing to prevent immoral acts of others, or giving or receiving orders that are perceived as gross moral violations. The act may have been carried out by an individual or a group, through a decision made individually or as a response to orders given by leaders.’ Indeed, commanders are not just responsible for the physical wellbeing of their soldiers, but through the moral consequences of their orders, their future mental health.

Some military leaders are disturbed by the findings, and say the term moral injury impugns the character of their soldiers. But researchers argue it’s quite the opposite: if soldiers felt nothing about taking the life of another human being, that would be indicative of sociopathy. Disturbance caused by killing indicates the presence of morality, not its lack.

Indeed, Maguen and Litz report, the combatant might see himself as ‘an evil, terrible person’ and ‘unforgivable’ because of acts done in war. Veterans might feel betrayed by the society that sent them to war or the superior officers who placed them in a situation where accidental killing of their own men or innocent civilians occurred.

‘When a leader destroys the legitimacy of the army’s moral order by betraying “what’s right”,’ writes the psychiatrist Jonathan Shay, an expert in combat trauma, in his book Achilles in Vietnam (1994), ‘he inflicts manifold injuries on his men.’ Returning vets who have killed are far more likely to report a sense of alienation and purposelessness caused by a breakdown in standards and values. They withdraw from or sabotage relationships. The sense of self-condemnation, those feelings of guilt, betrayal and shame, might remain hidden inside the warrior’s head until he returns home, and once the Trojan horse is safely inside the gates of Troy, the agents of destruction are unleashed.

This could be what happened to Corporal William Wold, who, not unlike Homer’s Achilles in the Iliad, was a brave and accomplished warrior made vulnerable by a fatal flaw. Wold’s mother Sandi said he was fine for a while when he first got home, but after a few months the darkness seeped out. He couldn’t eat and he never slept.

The transgression that bothered him most wasn’t the carnage in the mosque, but another, even more disturbing incident, an accidental killing at a vehicle checkpoint in Iraq. The vague description Sandi gave to a local television reporter is horrifying: ‘A vehicle came through that hadn’t been cleared,’ she said. ‘The lieutenant says: “Take them out.” He took them out. They went to the van – it was a bunch of little kids. And he had to take their bodies back to the family.’

It was in the calm of these ‘safe’ surroundings that his guilt and shame overwhelmed him

Instead of killing an armed enemy, Wold had, through the orders of an officer, killed several children. Accidental killing of civilians in the Iraq War, as in all wars, are much more common than you can imagine. Numbers are so high it wouldn’t benefit the military to keep accurate tabs; rigorous documentation would just fan the public relations nightmare and boost the propaganda value of the deaths for the other side.

Wold, like many combatants, was able to contain his guilt while still in Iraq. But when he returned home, he brought the Trojan horse with him. It was there, in the calm of these ‘safe’ surroundings, that his guilt and shame overwhelmed him. He became addicted to the pain medication prescribed for an injury he had suffered in a roadside bomb attack and augmented that with methadone that he scored on the street.

It was clear to his family that Wold was deeply troubled. They took him to psychiatrists, psychologists, tried everything, but nothing seemed to help, and he was unable to find any peace in civilian life. Though his mother begged him not to, Wold ultimately rejoined the Marines. ‘My brothers will take care of me,’ he said.

But when the Marines discovered his drug problem, they sent him to a treatment programme. When he failed to complete the programme, he was sent to a naval hospital near San Diego, to await his discharge.

One night a couple of friends came to visit Wold there. They went out together to see a movie and get tattoos. When they returned to his room, Wold couldn’t remember if he had taken his medication or not – so he took it again, in front of his friends. They watched TV for a while. The friends left when Wold fell asleep, but had plans to return in the morning to take him on a camping trip.

The next morning, the friends found Wold in bed, in the same position he had been when they had left him the night before. Only now he wasn’t breathing. They began CPR and called the medical staff to try and revive him.

He was pronounced dead at 9:35am. The date was 10 November 2006, just two years to the day I had talked with him against that shattered wall in Fallujah – and also, the date on which the US Marine Corps annually celebrates its founding in 1775.

The medical examiner’s autopsy stated that the cause of death was drug toxicity likely caused by the methadone Wold had added to his mix of prescription drugs; the brew probably led to respiratory failure, and death.

Sandi felt the Marines had failed her son. But she knew he had loved the camaraderie of the corps and had him buried in his dress blues. She also knew that the uniform was just the surface of a much more complex story, a story of belief, duty and honor yes, but also about how guilt over killing in the pursuit of those ideals could lead to ruin.

Both parts of the story were imprinted on Wold’s skin. On the inside of his right forearm was the tattoo that he had gotten the night before he died, an exuberant design of a woman and an eagle wrapped in a flowing American flag with a banner that read: ‘All American Bad Ass’. But a second tattoo, this one on the right side of his chest, had a more sombre message, an image of a pair of praying hands with the words, ‘Only God Can Judge’.

It is that sense of violating one’s own basic moral values, of transgressing against what is right, that separates moral injury from garden-variety PTSD. Today’s standard treatment for veterans suffering from combat-related PTSD involves prolonged cognitive and psychodynamic therapies where subjects either tell or write their stories over and over in an effort to bring context and reason to their experiences. This is done in a clinical setting, but it is also a nod to the value of the age-old practice of storytelling, especially within warrior societies, as a method for sharing both the burdens and the glories of war – like the Greeks with their epic poems, or Native American tribes of the plains speaking around their campfires, or Maori warriors tattooing their battle exploits on their bodies. Litz calls these evidence-based treatment therapies ‘so extraordinarily effective that it should be considered malpractice not to use them’.

But to treat moral injury, which can and often does co-exist with PTSD, the VA is testing a different approach: a six-session pilot treatment programme, currently run by Maguen, called Impact of Killing in War, or in the military world of forced acronyms – IOK. Silly acronym or not, the programme represents a seismic shift in the treatment of war trauma, embracing for the first time the concept that real healing might need to include moral and spiritual notions such as forgiveness and giving back.

The first step in IOK involves education; veterans literally learn about the complex psychology of killing in war and the inner conflict it provokes. Then, looking inward, they are trained to identify those feelings in themselves. The third step involves the practice of self-forgiveness. Finally, the veterans are asked to make amends through individual acts of contrition or giving back.

Keith Meador, a psychiatrist with a pastoral religious background, has been breaking down the barrier between mental health and spiritual care to help the veterans heal. His programme at the Durham VA Medical Center in North Carolina is tagged with yet another acronym – Mental Illness Research, Education and Clinical Centers, or MIRECC. ‘The piece that is particularly relevant,’ Meador told me, ‘is that patients don’t present to us saying this is my mental health need or this is my spiritual need. They come to us saying, “I’m suffering”.’

In the truest warrior tradition, he shared his story as an act of faith and an act of healing

A few small studies and reports suggest that the new therapy helps. Indeed, if Corporal Wold is our allegorical Achilles, felled by an untreated moral injury, then Lance Corporal James Sperry is our Odysseus, who, after struggling for years, finally makes it home.

I met Sperry, like Wold, during the Battle of Fallujah in Iraq in 2004. I videotaped him after he had been wounded during the first day of fighting. Like Wold, Sperry came home with a head battered from war and filled with guilt. But Sperry’s guilt wasn’t over killing; it was over not being killed, survivor’s guilt. His unit suffered some of the highest casualty rates of the war.

He sent me an email six years after Fallujah, thanking me for helping carry his stretcher that day and asking if I had any photos of his comrades killed in action. ‘I was the Marine that you helped carry to safety after I was shot by a sniper,’ he wrote. ‘I was wondering if you had taken any photos of me during that time of injury and any of my fallen friends. I have lost 20 friends in this war and would like to get as many pictures as I can.’

That note came during a dark period of Sperry’s life when he was struggling with cognitive impairment and debilitating migraines from his physical injuries and a host of psychological issues consistent with moral injury. He met nearly all of its criteria, including purposelessness, alienation, drug and alcohol use, and even a near-suicide attempt (he went as far as to sling a rope over the rafters of his garage).

His recovery, which took years, was not the result of a single act, but encouragement from family and friends, ongoing determination and a groundbreaking programme from the Shepherd Center in Atlanta, which specialises in helping those with brain and spinal cord injuries. That rehab blended the best traditions of Eastern and Western medicine, using yoga, acupuncture, hypnosis, psychotherapy and exercise. Once the myriad of prescribed medications he’d been taking had been dialled back, the fog that had enshrouded him for so many years began to lift.

Sperry did one more thing. He broke the silence. He shared his story with me for my book The Things They Cannot Say, with all of its setbacks, dark moments and eventual successes. In perhaps the oldest and truest warrior tradition, he shared his story as an act of faith and an act of healing, to help him and others, both soldiers and society, better understand what comes home inside a warrior’s mind after war. His story caught the attention of President Barack Obama and the First Lady, and he’s been invited to the White House twice.

But that wasn’t enough. In the style of veterans undergoing IOK therapy, his struggles inspired a new sense of purpose, leading him to found The Fight Continues, an organisation dedicated to helping veterans make the transition home. It does this in part, by tapping into the idea of service. Sperry and other members were in Moore, Oklahoma assisting victims of the devastating tornado there last May.

Corporal Wold and Lance Corporal Sperry are just two of millions. According to US Department of Defense data, since 2001 about 2.5 million Americans went to war in Afghanistan and in Iraq, with more than 800,000 deploying more than once. Nearly 700,000 of those veterans have already been awarded disability status, with another 100,000 pending, according to the VA.

They all need support. As Jonathan Shay wrote in Achilles in Vietnam: ‘When you put a gun in some kid’s hands and send him off to war, you incur an infinite debt to him for what he has done to his soul.’

We might do that best by anticipating what is coming home with them. If we can become more thoughtful about the consequences of conflict, the agents of destruction might someday be crowded out by the agents of hope inside the hollow horses pulled through our gates.