When soldiers kills someone at close quarters, how does it affect them? This most challenging and traumatic part of a soldier's job is often wholly overlooked.

Soldiers kill. It goes with the job, and they do it on our behalf.

But it's an aspect of their work which is widely ignored - even by the soldiers themselves - and this can cause them great psychological difficulty, experts say.

We talk about destroying, engaging, dropping, bagging - you don't hear the word killing Lt Col Pete Kilner

"A central part of what we do with our careers is we kill the enemies of our country," said Lt Col Pete Kilner, a serving officer in the US Army who has done tours in Iraq and Afghanistan.

"So it's very important that we understand why, and under what conditions it's the morally right thing to do to kill another human being."

Lt Col Kilner also lectures at the West Point Military Academy. He calls himself a "soldier ethicist" and has talked with countless fellow soldiers about their experience of "intimate killing" - taking the life of someone up close, who they can see.

"They don't like to talk about it. In general, if you're a soldier and you've killed in war, you lie and say no.

"It tends to be the secret we have that we're not proud of. We want to fight bravely, but it's hard to be proud of killing another person."

Image caption Learning to aim is one thing - learning to take aim at a person is quite another

Such acts are veiled by jargon, or not spoken about at all, he says.

"We recruit people to kill. We train people to kill. We make the orders. Yet after the fact, we don't talk about killing.

"We talk about destroying, engaging, dropping, bagging - you don't hear the word killing."

This revulsion against committing the ultimate deed prompts the question, how easy is it to do? Soldiers put on what some call their "warrior's mask" - but away from the heat of battle, how do they take it off again?

Experiences vary. Andy Wilson, a soldier in the SAS, Britain's elite special forces, joined the army at 18.

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Now 36, he still clearly remembers the first time he took someone's life in a kill-or-be-killed scenario.

"He had an AK47 and he was going to kill me. I was cool, calm and collected the whole time. I knew I had a job to do. I knew I was going to do it, and I did. I was a soldier. That was my job. And that was war."

Won't shoot

But what of those who refuse to pull the trigger? Military psychologists debate the issue of non-firers, and some say this is because their psyche is repulsed by the act of killing.

In World War II, SLA Marshall observed that many of his fellow soldiers didn't shoot. He wrote a study called Men Against Fire about this reluctance to kill the enemy.

Recalling the first time Ben Close, of the Coldstream Guards, first killed in Basra when he was 19 "A vehicle came towards our checkpoint and didn't stop, so I fired a warning shot at him. He put his foot on the accelerator, towards me and the gate. I didn't have time to think, my heart was beating really quick. Training took over. I just aimed up to his head, pulled the trigger. One round went through the windscreen - end of story. In that instant everything slowed down for me. I couldn't really hear much. The hardest time is doing it the first time. It got a lot easier after that."

"Fear of killing, rather than fear of being killed [was] the most common cause of battle failure," he wrote.

Marshall's research methods have since been questioned, but the broad conclusion is still accepted: soldiers often simply won't shoot.

The Reverend Dr Giles Fraser, who lectures on morality and ethics at the academy of the British Ministry of Defence, says there is a deep human reluctance to kill other people.

"Killing in combat for a psychologically normal individual is bearable only if he or she is able to distance themselves from their own actions.

"SLA Marshall found that only 15-20% of combat infantry were able to fire their weapons on the enemy and there were 80% that were de facto conscientious objectors when it came to the point of firing their weapon."

Lt Col Kilner, of the US Army, says the way to keep soldiers psychologically on an even keel is to reason with them - not to take away their choice and intellectual involvement with what happens in battle.

"If a soldier reasons that his or her cause is just, then killing sits more easily in the mind," he says.

Marshall's conclusions led the military to change the way soldiers were trained, to bring home the reality of confronting the enemy. For example, shooting practise no longer uses bullseyes, but human-shaped cut-outs that pop up unexpectedly.

"The experience of killing is huge and powerful. If you go in with the right personal tools, you can come out stronger. If you go in with cracks, you'll get shattered. The key is preparing people for this intense experience".