Nanking and Hobbes

The Human Animal creates invisible circles around groups and communities that give them identity. Moral codes and attitudes do not apply to Individuals from rival groups outside of this circle, who can in fact be imagined as sub-human or sub-animal. This is because it is necessary to destroy and eliminate communities that threaten the resources or safety of the group in question.
This is known as a Hobbesian Dilemma:

As a result, Hobbes believes that it is psychologically unnatural for altruists to exist. If just one narrowly self-interested person exists no altruist can survive unless he/she becomes narrowly self-interested too. In such an environment, known as a State of Nature, Hobbes argues that a person must always be suspicious that another will attack in order to maximize his/her own self-interest. Therefore, in order for a person to maximize his best interest, he must attack the other person before that other person can attack. Each such conflict between two people in a state of nature has been termed as the "Hobbesian Dilemma." However, in the field of Game Theory, the Hobbesian Dilemma has the same structure as a "Prisoner's Dilemma."

Hobbes believed that the "Hobbesian Dilemma" results in a State of Nature because morality is an unstable enforcer of social cooperation. According to Hobbes, a stable enforcer can only exist if not one person can deviate from the established rule by which the rest adhere to. Since cooperation among people is biologically necessary, a stable enforcer must exist. Hobbes believes that the best form of social enforcement is the existence of an all-powerful sovereign.

....which explains why governments exist and wage war with each other...because if there was no government, people would exist in a state of anarchy in which the Rape of Nanking would look like a mild argument. Funny how nature and biology are the driving forces in large abstract social concepts like "Nations".