Whether war is in conflict with the law of nature

Having seen what the sources of law are, let us come to the first and most general question, which is this: whether any war is lawful, or whether it is ever permissible to war. This question, as also the others which will follow, must first be taken up from the point of view of the law of nature.

Marcus Tullius Cicero,…following Stoic writings, learnedly argues that there are certain first principles of nature – ‘first according to nature’, as the Greeks phrased it – and certain other principles which are later manifest but which are to have the preference over those first principles. He calls first principles of nature those in accordance with which every animal from the moment of its birth has regard for itself and is impelled to preserve itself, to have zealous consideration for its own condition and for those things which tend to preserve it, and also shrinks from destruction and things which appear likely to cause destruction. Hence also it happens, he says, that there is no one who, if the choice were presented to him, would not prefer to have all the parts of his body in proper order and whole rather than dwarfed or deformed; and that it is one's first duty to keep oneself in the condition which nature gave to him, then to hold to those things which are in conformity with nature and reject those things that are contrary thereto.