The municipal law of Rome and of other states has been treated by many, who have undertaken to elucidate it by means of commentaries or to reduce it to a convenient digest. That body of law, however, which is concerned with the mutual relations among states or rulers of states (inter populos plures aut populorum), whether derived from nature, or established by divine ordinances, or having its origin in custom (moribus) and tacit agreement, few have touched upon. Up to the present time, no one has treated it in a comprehensive and systematic manner; yet the welfare of mankind demands that this task be accomplished.

Such a work is all the more necessary because, in our day as in former times, there is no lack of men who view this branch of law with contempt as having no reality outside of an empty name. On the lips of men quite generally is the saying…that, in the case of a king or imperial city, nothing is unjust which is expedient. Of like implication is the statement that, for those whom fortune favours, might makes right, and that the administration of a state cannot be carried on without injustice.