In The Coup de Grace, Bierce is saying that the nature of war is:
In the story, we are told that grievously wounded soldiers must continue to endure suffering for an indefinite period until they are tended to. This is because 'It is an army regulation that the wounded must wait; the best way to care for them is to win the battle. It must be confessed that victory is a distinct advantage to a man requiring attention, but many do not live to avail themselves of it.' The brutal reality of war is that even victory will not mitigate the severe loss of soldier lives.
Due to limited resources, the bodies of the fallen are often collected and deposited in crudely dug trenches. Those who die further away are buried where they lie. Bierce states that there is 'little attempt at identification.' The survivors have to make their way back to camp, helpless to aid the soldiers along the way who beg for water with their dying breaths.
In the story, Captain Madwell is horrified by Caffal's injury; in all his experience, he has never seen one like it. Bierce uses visual and auditory imagery to highlight the gruesome reality of Sergeant Caffal Halcrow's injuries.
Visual imagery: 'The only visible wound was a wide, ragged opening in the abdomen. It was defiled with earth and dead leaves. Protruding from it was a loop of small intestine.'
'In his giant agony he had torn up the ground on which he lay; his clenched hands were full of leaves and twigs and earth.'
Auditory imagery: 'At intervals he moved his limbs; he moaned at every breath. He stared blankly into the face of his friend and if touched screamed.'
'Articulate speech was beyond his power;...'
Bierce highlights the unforgiving nature of war by showing that war is no respecter of persons. In other words, rank or connection does not protect a soldier from grievous injury and/or death on the field of battle. Additionally, a superior officer may have to perform an agonizing and devastating act of mercy for those who have no will or ability to execute it.
Consciously or unconsciously, this writhing fragment of humanity, this type and example of acute sensation, this handiwork of man and beast, this humble, unheroic Prometheus, was imploring everything, all, the whole non-ego, for the boon of oblivion.
Captain Madwell spoke the name of his friend. He repeated it over and over without effect until emotion choked his utterance. His tears plashed upon the livid face beneath his own and blinded himself.
Captain Madwell mercifully shoots a horse whose foreleg has been splintered by a cannon shot. When he turns to Caffal, he discovers that he has used his last bullet for the horse and cannot perform the same act of mercy for his fellow human being. Drawing his sword from his scabbard, he has to brace himself for his difficult task. When he is done with his physically and emotionally draining task, he turns to face two attendants with a stretcher, and Sergeant Caffal's brother, Major Creede Halcrow. Now, he has to endure the unforgiving task of explaining his actions to the Sergeant's brother.