The recent conviction of a Royal Marine for the murder of a wounded Taliban fighter has brought to the fore the question of when a combatant should be treated as a prisoner of war and viewed with mercy.

During World War I this line was crossed probably far more frequently than acknowledged by official versions of events.

To an extent this is understandable. Soldiers would live for weeks, months, possibly even years with barely a glimpse of the enemy. Then, within a few minutes they could be in close proximity with the soldiers who they felt had been trying to kill them for so long.

In addition it must have been difficult for attackers in particular to react calmly when defenders attempted to surrender in the heat of battle – especially when they had just witnessed their friends and comrades being killed and maimed.

The idea of the very people responsible suddenly offering to surrender must have proved a difficult one for many soldiers to comprehend.

Various first-hand accounts describe the dangers involved for those trying to surrender or who had suffered wounds during an attack and were unable or unwilling to move.

Lance-corporal HC Lancashire of the London Regiment recalled an episode on 1 July 1916 (the first day on the Somme) in Martin Middlebrook’s book of the same name.

‘One of our bombers, ignoring instructions, pulled a hessian curtain aside in the entrance of a dug-out, instead of calling out first. As he bent down to call out, a German hiding behind the curtain delivered an upper-cut.

‘Infuriated that a German should punch him on the jaw, he screamed at him to come out. It was a low entrance, the German had to emerge bending down. The bomber raised his knobkerrie and felled the German with one blow.’

A knobkerrie was a stout stick with a spiked metal ball on the top. A blow to the head with such a weapon would have been fatal.

Prisoners were also killed in more cold-blooded circumstances than that mentioned by Lancashire.

Private TS Frank of the 2nd Battalion, the Green Howards, recalled: ‘In the heat of an attack, a German came running past us, surrendering. One of us dropped a grenade with the pin out into his wide pocket and waved him back. When it went off we all laughed, me included.’

A similarly chilling episode was mentioned by lance corporal JJ Cousins of the 7th Battalion, Bedfordshire Regiment.

‘We are filled with a terrible hate. Our actions are born of a terrible fear, the will to survive. Some of the Germans were getting out of their trenches, there hands up in surrender; other were running back to their reserve trenches. To us they had to be killed. Kill or be killed. You are not normal.’

Private JH Harwood of the 6th Battalion, Royal Berkshire Regiment, witnessed a similar episode at the Battle of the Somme.

‘I watched some Germans coming out of a dug-out and surrendering. They were holding up photographs of their families and offering watches and other valuables in an attempt to gain mercy but, as the Germans came up the steps, a soldier, not from our battalion, shot each one in the stomach with a burst from his Lewis gun.’

Even some distance from the frontline prisoners were not necessarily safe and could lose their lives for relatively trivial reasons. Lieutenant AW Lee MC saw the following episode unfold.

‘I watched our troops bringing German prisoners back, trying to make them walk in the open alongside a communication trench. The Germans kept going down into the trench. This annoyed their escorts so much that, eventually, they threw some grenades among the Germans and left them to it.’

Seen through the prism of 100 years such events seem truly dreadful, but war is a dehumanizing business. It is easy to be judgmental from the safety of one’s own living room – or courtroom – and less easy for the vast majority of us to understand the extremities of emotion experienced by those caught up in the immediacy and horror of close-quarter combat. As private Cousins observed about that day in 1916: ‘You are not normal.’