The efficient causes of war

As in other matters, so also in acts originating in the will, there are ordinarily three kinds of efficient causes – principal agents, auxiliary agents, and instruments. In war, the principal efficient cause is generally the person whose interest is at stake – in private war, the individual; in public war, the public power, in most cases the sovereign power. Whether war can be made by one on behalf of others who do not make war on their own account, we shall see elsewhere. Meanwhile we shall hold to this principle, that by nature everyone is the defender of his own rights; that is the reason why hands were given to us.

The efficient causes of war are in part those who wage war on another's account, as auxiliary agents

But to render service to another, so far as we can, is not only permissible, it is also honourable. Those who have written on the subject of duties rightly say that nothing is more useful to a man than another man. There are, however, various ties which bind men together and summon them to mutual aid. Thus those who are related by kinship unite to assist one another. Neighbours, too, and those who belong to the same state, call on one another for help.…But in default of all other ties, the common bond of human nature is sufficiently strong. Devoid of interest to man is nothing that pertains to man.