Realists are acutely aware of the immense costs of wars, especially the costs and risks incurred by the nation state they belong to. On the other hand, realists also recognize that there can be benefits of waging war in terms of territory and resources. Most importantly, realists claim that war and morality have nothing to do with each other; war is a non-moral activity.
Realists make two claims:
1) They argue that even in times of peace the relationship between different nations is not governed by any moral rules. In this sense nations are different from citizens living within a nation. So if a nation contemplates going to war moral considerations are beside the point
2) Once a war has started moral considerations remain irrelevant.
“In international society all forms of coercion are permissible, including wars of destruction. This means that the struggle for power is identical with the struggle for survival, and the improvement for the relative power position becomes the primary objective of the internal and external policy of states. All else is secondary” (N.Spykeman).
In a sense the realist argues that the international society is in Hobbes’ state of nature because there is no moral authority that can enforce moral rules, or to put it another way, there is no enforceable social contract among nations. So nations should only do what is in their own best interest in terms of survival. Another consideration is that if a nation is under the threat of war or is under attack, you do not have the luxury of rationally considering right and wrong. If your very existence is threatened the only option is to fight using whatever means necessary.
Just war theory. Just war theory distinguishes between two aspects of war: 1) jus ad bellum or the reasons why you go to war, and 2) jus in bellum or how you fight the war.
Just cause. You are only entitled to start a war in case A) National self-defense against an aggressor. B) In case one of your allies is attacked. C) For humanitarian reasons: if a minority in some country is suppressed or threatened with ethnic cleansing.
Legitimate authority. Only certain people or institutions have the authority to declare war, i.e., heads of state, parliament etc.
Right intentions. When you go to war you can’t change the original intention that gave you just cause, for example to defend your borders, and go on a war of conquest for new territories.
The likelihood of success. You should not engage in a war where your original objectives and intentions have little chance of success. So, if no good can come from a war, don’t go to war.
Proportionality. The costs of fighting a war have to be commensurate with the benefits you can obtain by fighting the war.
Last resort. You should only engage in warfare when all other peaceful means of resolving a conflict have been exhausted.
There are two main principles that hold for jus in bello:
Proportionality. The costs of any military action have to be proportional to the military objectives that can be accomplished by the action.
Discrimination. You should never intentionally target non-combatants, and as far as possible avoid putting them in harms way.