War may rightfully be undertaken on behalf of subjects

In the earlier part of this work, when we dealt with those who wage war, we asserted and showed that, by the law of nature, each individual was justified in enforcing not merely his own right but also that of another. The causes, therefore, which are just in relation to the person whose interest is at stake are just also in relation to those who give assistance to others. Now the first and particularly necessary concern is for subjects, either those who are subject to authority in a family, or those who are subject to a political authority. They are, in fact, as it were, a part of the ruler.

Refraining from war on behalf of subjects

Nevertheless, wars are not always to be waged on behalf of subjects, even though the just cause of some subject places the ruler under obligation to undertake them. Such wars are to be undertaken only when this can be done without loss to all the subjects, or to the majority of them. The duty of the ruler concerns the whole rather than parts; [although] the greater a part is, the more nearly it approaches the character of the whole.