The effects of a public war

[A] war declared between two peoples, or the heads of two peoples, has certain particular effects which do not arise from the nature of war itself.

Variant meanings of the word ‘permissible’

[S]ometimes that is said to be permissible which is right from every point of view and is free from reproach, even if there is something else which might more honourably be done.…Thus it is lawful to contract marriage; but, for a holy purpose, the chastity of celibacy is more worthy of praise.…Also, to marry a second time is lawful; but it is more honourable to be content with one marriage.

In another sense, however, something is said to be permissible, not because it can be done without violence to right conduct and rules of duty, but because among men it is not liable to punishment. In this sense, fornication is permitted among many peoples. … In this sense we often see what is permitted contrasted with what is right.

Public war as granting impunity for certain acts

With this restriction, therefore, it is permitted to harm an enemy, both in his person and in his property; that is, it is permissible not merely for him who wages war for a just cause, and who injures within that limit – a permission which we said at the beginning of this book was granted by the law of nature – but for either side indiscriminately.