A U.S. Marine carries an M240 machine gun during a patrol northeast of Marjah, Afghanistan, on Feb. 14, 2010. (Patrick Baz/AFP/Getty Images)

War is such a constant in the American experience that most of us are all too familiar with the evolving names we have given its emotional consequences in the past century: Shell shock, battle fatigue, operational exhaustion, and most recently, post-traumatic stress disorder.

But most Americans are less familiar with a related, if distinct, affliction known as moral injury, with roots in foundational religious or spiritual beliefs violated during war. And increasingly, military chaplains are on the front lines, tending to these misunderstood wounds.

Psychiatrists have used the term since the 1990s, but the concept has only recently been the subject of serious research by clinicians, some affiliated with the U.S. Department of Veterans Affairs.

“We’ve come a long way in defining moral injury, but it takes a long time to develop a tool to measure it,” said Shira Maguen, a psychologist at the San Francisco VA Medical Center and one of those developing treatment models for moral injury.

Maguen has helped the VA define an event as morally injurious if it transgresses “deeply held moral beliefs and expectations.”

Chaplain (Col.) Thomas C. Waynick, a senior Army Family Life Chaplain at Fort Benning, Ga., who is helping to develop training for chaplains to treat moral injury, puts it this way: “In the course of doing what I’m called to do, what I know how to do, what I’m trained to do — I violate one of my core moral principles … and that can split the soul.”

Often a soldier faced with a life-or-death decision doesn’t have time to sort out right from wrong. The outcome of split-second decisions — driving past someone who needs medical help, shooting a civilian who fails to stop at a check-point — can lead to feelings of guilt and shame.

Unlike PTSD, which has roots in trauma-based fear, the trauma that leads to moral injury is most often rooted in something the victim did (or didn’t do). While the two are distinct, they can often share psychological turf.

“Something terrifying happens to you, and your fear system goes haywire,” said Rita Brock, director of the Soul Repair Center at Texas Christian University’s Brite Divinity School. “Often in the middle of being terrified, you do something you feel terrible for afterward. [PTSD and moral injury] overlap a lot, especially in war combatants.”

Because the research on moral injury is in its infancy, there are no data yet on the number of combat veterans who suffer from it. But many who study moral injury say the destruction of internal ethical fabric as a result of combat goes back as far as Ajax in the Iliad.

“Some of the towering figures in theology were trying to rebuild a moral system out of the ashes of war,” said Brock. “Moltmann, Barth, Tillich, Wiesel — their entire universe and moral systems had collapsed, and they were trying to build theologies adequate to live up to that.”

But modern population-centered warfare has also made it more likely that today’s combatants will find themselves in morally precarious situations.

“There are no long front lines,” said Nancy Sherman, a philosophy professor at Georgetown University and author of “Afterwar: Healing the Moral Wounds of Our Soldiers” (Oxford University Press, May 2015). “The city or village is the war zone today. Women and children are armed. Men are fighting without uniforms.”

At the same time, because of their proximity to civilians, U.S. combatants are subject to restrictions on the rules of engagement. “Greater troop restraint in that environment means greater risk to self,” Sherman explained.

Many combat veterans see additional risks even when the guns have quieted. Because they think reporting any psychological impact from combat can harm their careers, many troops seek out chaplains for confidential conversations.

Chaplains “are often regarded by the forces as ‘safe,’ ” said Sherman. “They won’t have a doctor’s appointment that shows up on their record. Chaplains are increasingly going out to forward operating bases so they can be right there for critical debriefings.”

In one sense, Waynick said, training chaplains to minister to troops suffering from moral injury is simple.

“It’s the role chaplains have always had,” he said. “Where there’s a spiritual existential crisis, no one else in the military system has the task to help with that.”

Ultimately, most moral injury experts agree, healing mandates forgiveness of some kind. Some want God to forgive them, others learn to forgive themselves. Still others can’t forgive themselves for taking another human life, so they angle for something else.

“Nothing will be able to restore the life I took, or fix what did,” said TCU’s Brock, channeling the mind of someone suffering from moral injury. “But if I come to accept it — this is who I am — I can build life without letting this event define me. That’s a kind of forgiveness.”

Tim Townsend is the news editor at Timeline.com. His book “Mission at Nuremberg: An American Army Chaplain and the Trial of the Nazis” (William Morrow, 2014) was published in paperback last month.