Should any American soldier be so base and infamous as to injure any [prisoner]. . . I do most earnestly enjoin you to bring him to such severe and exemplary punishment as the enormity of the crime may require. Should it extend to death itself, it will not be disproportional to its guilt at such a time and in such a cause… for by such conduct they bring shame, disgrace and ruin to themselves and their country. - George Washington, charge to the Northern Expeditionary Force, Sept. 14, 1775
This quote's about 235 years old, but it has no expiration date. The only problem I have with it is that I don't know if Washington is only referring to the lowest ranks when he says "soldier". It seems apparent to me that this admonition against the abuse of prisoners should apply throughout the chain of command.