World War II. This next World War contained a fresh degree of anger and moral outrage largely influenced by propaganda that made soldiers on the front line more fearful and appalled by the enemy than in the previous war.

There was a genuine hatred on both sides, particularly the Germans who had been brainwashed by Hitler's Nazi policies.

The psychology behind World War II was something that hadn't been encountered in any war before and it stood to reason that the soldiers who experienced it may have been more prepared to shoot to kill than their First World War counterparts.

In Germany there were men who were selected from a young age and sent to highly indoctrinated military schools, where they leared of the superiority of the Nazi regime and the wonders of Hitler whilst also being told of the inhumanity of the enemy they faced.

Psychologically, they were trained not to see the enemy soldiers as humans but as targets to eliminate to ensure the success and continued growth of the Third Reich.

It's hard to say if this actually worked in practice. We know that select groups of soldiers not always on the front lines, like the SS Einsatzgruppen had no inhibitions rounding up and killing Jewish people, Gypsies and other political dissenters at home.

But in warfare and the heat of battle, for the ordinary soldier, things were a lot different.

In 1978, Samuel Lyman Atwood Marshall, a former US Army Brigadier General turned journalist and historian, allegedly interviewed World War II soldiers and claimed that only 15 to 20% shot to kill.

His work formed the basis of this theory for many years until later historians dissected his data gathering and found it to be at best exaggerated and at worse completely fabricated.

It seems Marshall instead drew upon his own experiences in war and then tried to find information to back up his beliefs.

But despite this, his theory that many soldiers didn't shoot to kill has been corroborated by other studies.

There still may also have been a slightly higher rate of those willing to shoot to kill in World War II than in World War I.

Authors and historians now paint soldiers in the Second World War as suffering from a feeling of helplessness, removal of morality and loss of choice that led to more brutal wartime decisions.

The Vietnam War. The Vietnam War took place from 1955 to 1975. By then the US had adjusted its tactics from those it used when training soldiers during the two World Wars.

The army now looked at preparing them to shoot first and think later. This psychologically helped remove people's natural inhibitions to killing, conditioning them to react before their brains and any moral issues they might have kicked in.

This change in methods shows that even if we don't know how prevalent the problem was exactly, there was certainly a high enough percentage of soldiers not shooting to kill that the US felt it was worth addressing.

Historian Grossman believes that during the Vietnamese War those willing to shoot to kill increased gradually.

And by the end of the war in 1975, over 90% of troops had indeed shot to kill, a far higher percentage than any estimated during the world wars.

But at the end of the day, studies into whether or not soldiers shoot to kill are still inconclusive and often biased.

Specific battles are chosen to study to the detriment of others and whether or not shooting to kill means different things to different cultures is something that still hasn't been properly explored.

Plus, the psychological impact of a war zone and how it changes perspective skews results and even firsthand accounts.

In today's armies, training includes extensive dehumanization of the enemy and encouragement to see them as targets not people.

That's why in training targets are soldier shaped and often have identifiers that liken them to the real enemies that the soldiers will face.

Since the Second World War, something as simple as having soldiers fire at these types of targets as opposed to a round bullseye has greatly increased the number of them willing to shoot directly at an enemy on the battlefield.

Targets will also pop up at random moments and the trainee would be forced to react and shoot as quickly as possible, being rewarded for successfully hitting the mark.

This conditions the soldier to treat shooting an enemy as a matter of instinct rather than a conscious act.

Whether or not that leads to an increased number of soldiers willing to shoot to kill is hard to say, but many believe the numbers of those who are willing to kill have increased since Marshall's study of the First World War.

Still, overall, it's hard and nearly impossible to quantify the truth behind the statement shoot to kill and its reality in today's armies and the world.