Over at The American Scene, Jim Manzi wonders exactly which moral principles distinguish torture from brutal violence on the battlefield:
It can’t just be that it involves inflicting horrible pain and suffering. The moment before an enemy combatant surrenders, it is legal (under the current rules of war which govern U.S. military operations as I understand them), to shoot this person in head, launch burning petroleum jelly onto him that is carefully designed to stick to his skin and clothing, or deviously hide explosives that will maim him (but intentionally not kill him) when he steps on a landmine, in order to slow the advance of the group that must then carry him, and also to make it easier to subsequently kill both him and the person who assists him.
It can’t just be that the prisoner is helpless, or that the imbalance of power between the inflictor and recipient of the suffering is so high. The whole point of maneuver in warfare is often to put yourself in a position where you can cause massive causalities from a protected position. It is normally a retreating army that suffers the worst causalities. After all, it is legal (as I understand it) to drop a bomb from a virtually invulnerable aircraft at 30,000 feet onto an enemy combatant who has dropped his rifle and is running away at high speed.
John Schwenkler briefly makes the moral case with a comparison to counterfeiting:
Counterfeiting is wrong, of course, not because of the intrinsic properties of the pieces of paper involved, but because the practice of using as legal tender only those pieces of paper (or metal) that have the right sort of causal history is the very basis of modern economies. And in the same way, torture is wrong not because pacifism is true but because the practice of drawing bright lines to distinguish wanton cruelty from necessary evils is what makes human society the remarkable thing that it is....
Jim asks why “suddenly upon [an opposing combatant’s] saying the words ‘I surrender’, any serious violence beyond confinement becomes wrong”; the obvious answer, though, is that such violence is wrong because he’s surrendered, and that what sets off humans from other animal species is our recognition that, except perhaps when it is a matter of punishing someone pursuant to a conviction in an impartial court of law, killing people who have surrendered themselves is an immoral thing to do.
I think it's important to add, though, that there is also an enormously significant practical concern here, one that I would guess is at the root of the modern moral (near-)consensus. Specifically, when a group of combatants are badly outnumbered, or surrounded, or otherwise very, very unlikely to win a conflict, they have a considerable incentive to surrender--but only if they believe they will subsequently be treated with mercy. That is why individuals, and nations, surrender. If, by contrast, a group of combatants believes that, by surrendering, they are only making themselves vulnerable to further harm--specifically torture and/or death--they have no incentive at all to stop fighting.
The humane treatment of surrendered captives, therefore, is a crucial--arguably the crucial--understanding between adversaries if their conflict is to end in any way other than with the wholesale slaughter of the losers. It's worth noting, too, that it is not merely the lives of the losers that are preserved. If they do not surrender, it may be that they are all killed; but it is very likely that, in the process, they will also kill some, perhaps even many, of the eventual victors.
I am no military historian, but it's my understanding that many armed conflicts that we might consider pre-civilized concluded with just this kind of slaughter (and pillage, enslavement, etc.), and that the widespread recognition of civilized rules of war has saved literally countless lives. As bad as the Nazis were, I think it's unequivocally a good thing that we were not forced to depopulate Germany. The reason we weren't was that Germany surrendered, and the reason Germany surrendered was its well-placed faith that we wouldn't depopulate (or torture, enslave, etc.) the nation anyway.
And, yes, I recognize that not all modern combatants adhere to these rules. But Manzi, as I understand him, was asking why we have the rules in the first place.