On March 24, 2007, in a city just north of Baghdad, US Army infantryman Alex Horton shot a man twice in the abdomen. He saw the man stumble, then fall behind a building, out of sight. That’s all Horton knows, and all he will ever know, about what happened. He’ll never know if he was right to conclude, in that crucial split second, that the man was a threat to him and his fellow soldiers. But he thinks he was probably wrong. And that knowledge causes him lasting pain.

Horton's unit was on its 10th day in Baqubah, the capital of Iraq’s Diyala province, tasked with rooting out Sunni insurgents. They were out on patrol when a US Army Stryker traveling a few minutes ahead of Horton's ran over an IED. The explosion was so powerful that it knocked the vehicle on its side. Everyone in the Stryker was injured. One soldier’s leg was destroyed, shattered by the force of the blast.

Horton and the rest of his unit took up positions near the overturned Stryker, trying to secure the area until the wounded could be evacuated. Horton went up on a roof, where, as he remembers it, "we were just shooting hundreds of rounds down this road. Just suppressing anyone that's moving."

Horton caught sight of two Iraqis running across a nearby street, toward the fighting. The first man made it across the street and behind a building. Horton, believing they were a threat, aimed and fired, hitting the second man twice — Horton thought he saw one bullet hit his chest and one his side, though it was hard to tell in the seconds before he stumbled and disappeared from sight.

The man's momentum carried him forward and out of view as he fell. It was impossible for Horton to know whether he'd survived. The entire incident was over in a few seconds.

Horton knows why he made the decision to shoot: he was in the middle of a firefight, with wounded comrades to protect. The man he shot appeared unarmed, but he was acting in a way that seemed strange and, in context, threatening. At the time, in that split second, with less than a moment to decide, pulling the trigger seemed like the only option.

But in hindsight, Horton now believes his decision was wrong. By that point in the war, Horton told me, the insurgents fighting in Diyala were hardened professionals who wouldn’t have been dumb enough to run across a street that way.

"When you Monday-morning quarterback it," Horton said when we spoke in April, "it doesn't make any sense for someone who has ill intent to move like that." He now worries that the man he shot was a civilian, and posed no threat to him at all.

"That's not how good people act. But I did it, because I had to."

That belief has caused Horton a great deal of psychological pain since that day in Baqubah. His is a sort of battlefield injury that is increasingly recognized by psychological experts as being common among veterans. Though it can lead to symptoms severe enough to meet the definition of post-traumatic stress disorder (PTSD), it is its own distinct type of trauma. This is psychological trauma that results when someone violates his or her own moral code, or experiences a severe violation by a trusted figure, such as a commanding officer.

Psychologists call what Horton suffers from "moral injury," which they describe as an experience that violates core ethical and moral beliefs, thus leading to trauma. It's not the trauma of a physical threat to life or limb, but of damage to the "deeply held beliefs that undergird a service member's humanity," according to researchers at the National Center for PTSD.

Moral injury is not unique to veterans. But as psychologists and advocates come to understand it better, they are discovering that the problem is widespread in that community, and can be deeply damaging to veterans who suffer from it.

The existence of moral injury is troubling for other reasons, as well: it reveals deep tensions in the way we think about war, heroism, and what it means to do the "right" thing in battle — and serious questions about whether the moral demands that we make of our service members are reasonable.

The veterans who weren't getting better

Psychologist Jonathan Shay came upon the idea of moral injury in the 1990s as part of his work with Vietnam veterans. The war had been over for decades, but some veterans, he found, were not getting better. Shay eventually came to the conclusion that the root of his patients' pain was a kind of psychological trauma that had not gotten much study — the trauma of moral transgression.

The idea of a guiding morality, and that moral goodness will see one safely through war, is central to the US military's thinking and culture. In his groundbreaking 1994 book Achilles in Vietnam, Shay wrote that an army is in many ways a "moral construct": a social entity bound together by "shared expectations and values."

This idea of a moral code is part of how the military is able to function. As Shay wrote, "The moral power of an army is so great that it can motivate men to get up out of a trench and step into enemy machine-gun fire."

But that code's power is also a source of vulnerability. Soldiers put their trust in those shared expectations and values, but their trust isn't always rewarded. And when it isn't, that can cause a deep psychological wound.

Shay coined the term "moral injury" as a way to describe the traumatic feelings of betrayal and shame that many of his Vietnam veteran patients felt when the military violated the moral construct it had instilled in them. In his view, moral injury was at the heart of lasting combat trauma.

"Veterans can usually recover from horror, fear, and grief once they return to civilian life," he wrote, "provided that ‘what's right’ has not been violated." But when they believe that their commanders, or they themselves, have damaged the correct moral order, those psychological injuries can be much harder to come back from.

Shay focused on moral injuries that resulted from commanders violating that code — violating "what's right." One soldier, for instance, described the shame he felt after being awarded a medal for an incident in Vietnam in which his team mistakenly fired on a group of civilians unloading fishing boats. Although that soldier subsequently fought bravely in many other battles, Shay writes, he still felt "deeply dishonored" by the fact that he was awarded a medal for killing unarmed civilians. While the soldier himself clearly felt his actions in that battle were wrong, the fact that he was awarded a medal for it made it seem like the whole military structure was complicit in the moral violation, the betrayal of its fundamental values. And because those values were such a deeply held part of military life, that violation was devastating.

In the two decades since Shay's study of Vietnam veterans, the United States has gone to war in Iraq and Afghanistan, where it has deployed 2.5 million service members. As the wars wind down and most of those men and women transition back to civilian life, a number of them are experiencing something similar to what Shay discovered in his patients.

That has led to a renewed focus on moral injury. Brett Litz, a professor at Boston University and a Veterans Affairs psychologist who studies moral injury, told me that Shay's focus on commander betrayal was too narrow to fit the range of moral injuries veterans were experiencing during the wars in Iraq and Afghanistan. Litz said that others had to broaden the definition of moral injury in order to "operationalize" it for veterans who are suffering today.

Today, Litz and others believe moral injury can result from an individual violation of "what's right" or a betrayal by superiors. What matters, he told me, is the "transgression of deeply held beliefs," whether the service member transgresses his or her own beliefs or experiences a betrayal by a leader or comrade. Sometimes, Litz said, sufferers have committed an act that violates their moral principles, such as a killing. Sometimes it is a failure to act, such as failure to stop an insurgent who went on to kill a fellow soldier. Moral injury can also arise out of a sense of betrayal, if someone in a position of authority, such as a superior officer, violates shared moral principles that person was supposed to be safeguarding. It is marked by feelings of shame, guilt, and self-blame. In its more severe forms, it is often accompanied by self-destructive behaviors and can lead to suicide.

No epidemiological studies have yet measured the frequency of moral injury among veterans or the broader population. But some research suggests it is very common among those suffering from battlefield trauma. Litz told me that he recently conducted two studies, one of Marines and one of soldiers. He found that 30 percent of those who sought treatment for PTSD indicated that their trauma arose out of moral injury, rather than the experience of physical danger.

Moral injury: the pain of transgressing your own beliefs

Moral pain, like physical pain, can fall on a spectrum from mild to severe. We have all experienced some version of it. Everyone knows what it feels like to do or say something cruel and then burn with shame and remorse. That feeling can be useful: it helps us do the right thing in the same way that physical pain helps us protect our bodies from harm. But just as physical pain can become overwhelming if the injury is severe or the pain chronic, moral pain, too, can become a debilitating burden.

Horton's example is instructive. He is high-functioning by any definition, an accomplished writer and an adjunct lecturer at Georgetown University, in addition to his primary career in communications at a nonprofit. He has not suffered the kind of debilitating psychological pain that is associated with pop culture portrayals of veterans with PTSD. Rather, he said, he experiences moral injury as a feeling of self-doubt and self-criticism that is "always humming in the background" — a sort of psychological tinnitus.

"You compare yourself to all those moments," Horton told me. "You shouldn't, because it will keep you up at night."

The problem, for Horton, began when he returned from Iraq. As the feelings of instinct and urgency that had driven him on the battlefield fell away, he came to look at his actions during the war very differently. He often focused on the man he shot that day in Baqubah, but there were other, more minor incidents that gnawed at him, too, such as the way he behaved toward Iraqis during house raids.

There is one moment in Diyala that stays with him. He was trying to enter an Iraqi home after his unit had been ordered to take shelter there, but a man came out and barred the gate with his arms, blocking Horton's way.

"I don't even miss a beat. I grabbed his arm and I just threw him to the ground, without thinking, without considering it, anything."

Compared with the casualties and violence his unit faced, that was a minor incident, "almost a non-event." But later, it came to bother him.

"Time's finally up," Pierce's suicide note read. "I have taken lives, now it’s time to take mine."

"I remember it clearly, because I'm not someone who just goes shoving people out of the way to the ground. Saying with my actions, 'You don't matter. You are in my way. You are a barrier to overcome,'" Horton said.

"That's not how good people act. But I did it, because I had to. Every second I spent on that street was a second I could get shot or someone I know could get shot. I remember that very clearly. Even though it's a very small thing, it mattered after that."

But when the transgression is more severe, the psychological pain can be, as well. In 2007, a 23-year-old former Army infantryman named Noah Pierce parked his pickup truck at a small clearing near his home in Minnesota, drank part of a bottle of Jack Daniels, and shot himself in the head. Next to him were his photo IDs, each of them stabbed through the face, and a suicide note. "Time's finally up," it read. "I am not a good person, I have done bad things. I have taken lives, now it’s time to take mine."

Pierce, Ashley Gilbertson wrote in a moving profile in the Virginia Quarterly Review, had been diagnosed with PTSD, and it is difficult to know what the precise issues were that motivated him to take his own life. But he had clearly been tortured by his actions in Iraq. In 2005, he had run over an Iraqi child with his Bradley fighting vehicle. "I feel really bad but I thought he would get out of my way," he wrote in an email home to his parents.

In another message home, Pierce implied that he had shot an Iraqi soldier at point-blank range, even though the man was on his knees surrendering. He wrote, "Say you just shoot out of instinct like hunting, like when you suddenly flush a grouse (dad should know what I mean). Then, after, you realize what you did. Is that considered murder?" Later, Pierce shot the driver of a car that failed to slow down at a checkpoint, but then discovered the man was a doctor, not an insurgent.

One day, during the months between Pierce's return home and his death, his mother found a note he had written to the Iraqi doctor he had killed, begging for forgiveness. In the note, Pierce was "apologizing over and over," she told Gilbertson. "'I am so sorry. I am so sorry. Can you ever forgive me?'"

The lie that combat is morally safe

When you join the United States military, you are inculcated with a set of moral values meant to guide you through the hardships of war.

Patricia Resick, a psychologist and researcher who works with patients suffering from moral injury, told me soldiers are taught that "if everybody does their jobs correctly, everyone will come home okay." That seemed to be a function of modern military training and culture, she said. "I don’t think the people who were landing on D-Day expected to survive." But today, "it is either explicitly or implicitly taught that if they just practice enough, and if they do enough and have the right weapons, they will come home."

That idea, Resick believes, makes it easier for service members to go into battle by giving them an illusion of control, or at least that by having true and good values they will be more likely to survive. The alternative is to accept "that somebody else had control in that situation, or some other forces had control in that situation," and that is too frightening to countenance.

But the reality of war is often messier than the world of moral black and white that service members are asked to adopt. At times, that can force them to violate their military-ingrained values in order to survive. And any decision to knowingly violate one's moral code, no matter how understandable it may seem to outsiders, can be deeply traumatic.

"We teach our Soldiers, Marines, Sailors and Airmen values that are often more rigorous than what they grew up with at home or in school," Steven Dundas, a Navy chaplain who suffers from moral injury, wrote in an essay on his personal website. But in battle, "the moral values we teach them are contradicted by what we teach and train them to do, and the real unvarnished truth about war, it is hell."

As former Marine Stephen Canty told the Huffington Post, once someone has learned to kill, "What is morally right anymore? How good is your value system if you train people to kill another human being, the one thing we are taught not to do? When you create an organization based around the one taboo that all societies have?"

Nancy Sherman, a professor of philosophy at Georgetown University who is the author of Afterwar, a new book about moral injury, pointed out that this moral code often becomes idealized into a mandate: "be the best," "always bring your troops home," "never leave a fallen comrade behind."

Soldiers have come to believe that combat can be morally safe: that if they do their jobs, then they will not be forced to face situations in which they must violate their deeply held beliefs, because their military and their society have told them over and over that combat — and life — works that way.

But the truth, of course, is that "everybody can do their job and a bad thing can still happen," Resick points out.

The military's rigid idealism doesn't adapt well to the realities of battle when not everybody comes home. Survivors, then, often look inward for sources of responsibility, out of a belief that because they were told being moral would make everything okay, if something bad happens it must be because they failed to live up to their moral requirements. In response, soldiers may torture themselves with blame.

The moral danger of the battlefield

Imagine you are a soldier in Iraq or Afghanistan. Somewhere, perhaps while on patrol, your team is surprised by a boy who seems to be carrying a gun or a grenade, and to be approaching with hostile intent. In that moment, your only clear choice to protect your fellow service members is to shoot the boy. If you fire, you will kill a child; if you don't, a fellow soldier whom you are responsible for protecting may die. In making either choice, you will knowingly violate your fundamental moral code: killing a child is a deep moral taboo in any culture, but in the military, a failure to protect your fellow soldiers is an equally devastating transgression. No matter what you choose, you will live the rest of your life with the weight of that decision.

It is nearly impossible to prepare for that type of moral decision, one in which there is no way to do the "right" thing without also doing something "wrong." Sherman described such circumstances as a "contingent conflict": a situation in which making any choice, even for good reasons, carries a serious moral price.

Those decisions fall outside the normal rules of our moral universe. From childhood, we are taught that being a good person means doing the right thing even if it is difficult or unpleasant. That's the message of every fairy tale: that virtue is about self-sacrifice and steadfastness. But on the battlefield, different rules apply.

"Having to shoot a child to save your fellow soldiers or Marines — there's a lot of dimensions to that"

As in the hypothetical example above, there may be no way to do the right thing without also doing wrong. Those are the true moral dilemmas of combat: balancing between conflicting moral goals, such as the decision about whether to kill a child or risk the lives of one's fellow soldiers.

Such situations, in which a soldier feels forced to do something that he or she finds morally abhorrent, are a "classic example" of moral injury, said Molly Boehm, a researcher at the Center for New American Security.

"Having to shoot a child to save your fellow soldiers or Marines — there's a lot of dimensions to that," she said. "It's not just having to take the life of a child. It's also that the enemy is such that they have violated what is our moral code. They have allowed or forced a child to be a soldier, which in turn means I was put into a position where I had to kill him in order to protect my buddies."

Resick said soldiers often impose a heightened sense of responsibility on themselves, feeling guilt and shame for failing to save a fellow service member, even if doing so would have required them to disobey orders or leave their post. In many situations, then, the moral demands that a soldier places on him or herself are unrealistic or impossible to meet.

Forcing veterans to pay the price for our comforting moral illusions

There is another reason soldiers are so destabilized by the moral contradictions of war: it is totally at odds with how we are led to believe the world works.

We, as a society, tell ourselves the world divides neatly between right and wrong, between good people and bad people, and that goodness is rewarded and evil punished. Although we may believe we leave such simplistic moral views behind when we reach adulthood, that's not really true. Deep down, we still expect to be rewarded for goodness. Why else would we so frequently respond to tragedy or pain by exclaiming, "That's not fair!" or "That's not right!"?

This perspective has a name — social scientists call it the "just-world" belief or the just-world myth. Resick said people are drawn to this idea because it gives them a sense of control over an unpredictable world. The alternative, acknowledging that bad things can happen to anyone, or be done by anyone, is too much to bear. Researchers have found that belief in the just-world myth is especially strong in the United States. Indeed, many of the ideals of American culture — that hard work will lead to success, that poverty is the result of laziness or poor moral fiber, and that success is due to merit, rather than luck or unfair advantage — are rooted firmly in just-world beliefs.

The allure of the just-world myth is especially strong when it comes to war: our troops are the good guys, and if a soldier has sufficient strength of will and character, then he or she is only capable of doing good. Criticism of our soldiers, or portrayals of them as morally flawed, are unacceptable. Even politicians who oppose war still feel it's necessary to say they "support the troops." We put service members on a pedestal as morally uncomplicated heroes.

This view of the world reassures us that if our intentions and values are true, our actions can only be good. But it is a lie, and it is a lie that we implicitly demand veterans uphold for us when we send them into situations fraught with moral danger and then ask them to come home and be reassuring moral paragons. Their moral injury is the price we ask them to pay to maintain our fantasy.

This myth and its damage extend far beyond war. This same desire to believe in a just, controllable world causes people to blame rape victims for their own sexual assaults, for instance. "Look how they’re treated," Resick said. "'You caused it. You did something wrong. You must have done something to bring this on. You wore a short skirt, you had a drink, you went into a bar.'" It's too frightening to imagine that sexual assault could happen to anyone, so we look for a way that the victim's behavior deviated from what was "right" or "good."

Treating the gray areas in morality

One of the therapeutic techniques Resick uses for moral injury starts with a pie chart. If, for example, a patient blames himself for the death of a fellow soldier, his therapist might ask him to draw a pie chart apportioning responsibility for the incident.

"We put it in a pie chart so that we don’t get up to 300 percent. There’s only 100 percent of guilt and blame to give out," she said.

"When people first start out and they have PTSD," Resick said, they begin by assigning themselves 100 percent of the blame, or perhaps 50 percent to themselves and 50 percent to the unit commander.

"And then the first question the therapist would ask would be about the insurgent. And then they say, 'Oh, well …' And then we have to start over with a new pie chart." That a therapist would need to force such a realization on the patient at all speaks to the degree to which self-blame can come to define the inner life of a moral injury sufferer.

That treatment process can be very difficult. The constant self-condemnation moral injury sufferers feel can seem insurmountable, like a wall preventing them from moving forward with their lives.

Patients want to atone for their actions. Therapists can help them do that in a constructive way.

Therapists treat moral injury by carefully chipping away at that wall. Resick uses a method she developed called Cognitive Processing Therapy (often abbreviated to "CPT"). She described an approach to therapy in which therapists gently argue with the logic of self-blame, urging patients to evaluate themselves using less rigid moral standards.

A cognitive processing therapist, she said, works with the patient to question every step of the logic of self-condemnation. If a veteran insisted that he was a murderer because he shot a civilian whom he mistakenly believed was a threat, for instance, Resick said she would demand that he go back to his reasoning in the moment. "What did you know? What did you think at the time?"

Litz is developing another type of cognitive therapy for moral injury called Adaptive Disclosure Therapy. In that treatment, veterans imagine disclosing what they did to a compassionate moral authority: someone who would hold them to appropriate moral standards, but who also cares for them and sees them as a whole person. A loving parent, for example.

The therapist guides the process as the patient creates the imagined dialogue, which culminates in a plan for reparations and redemption. The idea is that patients feel a need to atone for their actions, and therapists can help them do that in a healthy, constructive way, for example by doing community service. This may be tailored to the nature of the injury: for instance, a patient who feels guilt over killing a child may volunteer his time with needy children in the US as a way of paying off the perceived moral debt.

The truth about war that we refuse to see

By its mere existence, moral injury requires us to admit that our vision of war, the way it is portrayed in our pop culture, is an idealized fiction. The vision of war implied by moral injury — corrupting, full of uncertainty — is simply not the story Americans want to tell themselves about war.

In the past, films such as M.A.S.H., Platoon, and Apocalypse Now have portrayed the Korean and Vietnam wars as morally compromised and compromising for the soldiers who participated. Now, though, Americans gravitate toward first-person shooter video games, and to more black-and-white war stories such as Lone Survivor and American Sniper, the latter of which is now the most successful war movie of all time after earning over half a billion dollars at the box office.