On good faith with enemies in general

We have said that, in respect to character and extent, what is permissible in war is considered either absolutely or with reference to a previous promise. The first part of the subject has now been finished; there remains the latter part, which concerns the good faith of enemies with one another.…Those who are enemies do not in fact cease to be men. But all men who have attained to the use of reason are capable of possessing a right which has its origin in a promise.

From the association of reason and speech arises that binding force of a promise with which we are dealing. Because we have previously said that, in the opinion of many, lying to an enemy is either permissible, or free from wrong, it must not be thought that this view can be extended with like reason to pledged faith. For the obligation to speak the truth comes from a cause which was valid before the war, and may, perhaps, in some degree, be removed by the war; but a promise in itself confers a new right.

Whether faith should be kept with pirates and tyrants

Already in our previous discussion we have said that we ought not to accept the [contention that good faith is not owed to pirates and tyrants and the like]. … Such agreements [admittedly] do not in fact share in that special community of legal obligations which the law of nations has introduced between enemies engaged in a formal and complete war.