You have only three seconds to decide what to say. An angry soldier in front of you is about to shoot an unarmed prisoner. What words can you use to stay his itchy trigger finger? This is a question used by a philosopher employed by the US navy in officer training. The trainees are told they have time to say only one thing.
Not surprisingly, it is not "Paragraph 4 Subsection 2 of the rules of engagement forbid you from doing this". The recommended thing to shout, according to the professional ethicist, is: "Marines don't do that." We will come to why later.
The sad fact, of course, is that some Marines do do exactly that. On Friday, a court martial will meet to sentence the sergeant from the Royal Marines who was found guilty last month of murder under Paragraph 4 Subsection 2 of Section 42 of the Armed Forces Act 2006. The first British serviceman to be convicted of murder on overseas duty since the Second World War had come across an Afghan insurgent who had been seriously injured by gunfire from an Apache helicopter sent to provide air support for the Marines' patrol.
Footage from a helmet camera video of the killing showed the commando shooting the man point blank in the chest with the words: "Shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us." He then said to his comrades: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."
The response, among soldiers and civilians, has been polarised. Major-General Julian Thompson, who led 3 Commando Brigade during the Falklands war, urged a lenient sentence of no more than five years because of the "unique pressures of war". But General Lord Guthrie, a one-time army chief of staff, said "murder is murder" and urged a tough stance against this "battlefield execution". Pleas for clemency erode our "moral ascendancy over our enemies", said General Sir Nick Houghton, the current Chief of the Defence Staff.
Battlefield ethics draw on two ancient traditions. The Stoics advocated detachment to make a soldier strong and self-sufficient. There is no place for emotions such as anger, grief or other feelings that make a warrior vulnerable when he has to stare down death.
Aristotle, by contrast, says that anger is a proper response for a soldier because he remains a man and a social creature. Righteous indignation is a good thing so long as we do not become slaves to our own anger. Balance is crucial, as in all things.
The British and US armies work on a combination of both philosophies. In a world which is increasingly self-focused, hedonistic and utilitarian, the military has been one of the last bastions of Aristotle's view that we act morally out of character, or habit, which comes from the right training by our parents and teachers.
Army discipline is the acme of that. Men are repeatedly drilled so that when action comes they will react as they have been trained to do. "We become by doing," says Nancy Sherman, professor of philosophy at Georgetown University and former chief ethicist at the US Naval Academy. "You need to rely on character."
"The system was about inculcating that and giving it a moral inflection," says the Rev Dr Giles Fraser who has taught ethics at the British Defence Academy at Shrivenham. "Through the heroes of the regiment, the pictures around the mess wall, the regiment becomes a sort of moral idea. So the words 'Marines don't do that' takes on a moral meaning."
Yet over the past century, morality has become increasingly rules-based. "The modern battlefield is so fluid, fast moving and chaotic that a rules-based approach breaks down," says the Rev Fraser, who taught philosophy at Oxford. "We've let law shoulder the burden of ethics, and that's no good when you've got a split second to make a decision."
But there is a problem with relying too much on character. Training hammers into soldiers not that they are not fighting for Queen and country, nor freedom and democracy, but out of loyalty to the sacred band of brothers alongside them.
There was a moving moment at the last Remembrance service where a war widow described how her husband drew the fire of the Taliban so the men in his platoon could escape an ambush. But it is out of that same loyalty that grows the instinct for revenge which appears to be what motivated Marine A to finish off his captured enemy.
Character internalises our sense of what is right. So if battle skews that then our moral compass points awry. Pious moralising about this "not being a killing in the heat of battle" underestimates the changes wrought by fear and stress – or by seeing the Taliban use body parts of fallen British soldiers to entice their comrades into booby traps. Unlawful killings on the battlefield most often occur when soldiers have lost a friend. Marine A had lost seven.
Revenge is despised as a primitive emotion. Professor Sherman is not so sure. To her, revenge is part of "an unspoken pact an soldier makes with himself in preparing to face the enemy". It may practically be impossible to separate revenge from solidarity with a soldier's comrades.
There are other considerations. Why is a Marine who kills one man tried when those who direct drone strikes that kill scores avoid the courtroom? And what about the politicians who give the orders? This is not to exonerate or make excuses. But it is to demand that society bears some responsibility in the acts of men who have been brutalised in its service.
Marine A had done seven tours of duty in Northern Ireland, Iraq and Afghanistan. He was experienced enough to know better, but he was traumatised enough to lose his moral bearings momentarily.
War is not over when fighting ends. Last year, more British soldiers and veterans took their own lives than were killed in battle. What is clear is that the judges on Friday will have before them a killer who is far from a common criminal. War cannot be one soldier's private burden.
Paul Vallely is visiting professor of public ethics at the University of Chester