Commentary on the Law of Prize and Booty. Hugo Grotius

      Accordingly, if the law of nations is taken as a criterion rather than some civil precept, faith must be kept with the enemy in every way (as Cicero^a maintains), and even (so Ambrose^b specifies) when one is dealing with a treacherous enemy. It should be understood, however, that the foregoing doctrine is dependent upon the supposition that the enemy has not previously departed from the particular contract^c that is the basis of the required good faith;^d for in such cases it will be evident, inasmuch as the obligation is mutual, that the terms of the promise have lapsed, so to speak. Apart from this one exception, it may be said that,

      He is the best of soldiers who aims first

       And last, ’mid wars, to keep good faith intact.^e

      Neither, then, can fear be accepted as an excuse, since even he who has made a promise in order to escape misfortune cannot deny that he himself chose this course of action in preference to the alternative course. In short, will that is thus coerced nevertheless retains its voluntary character and, once it has been expressed (albeit to an adversary), has binding force. With respect to this point, we should abide by the opinion of the theologians^f rather than by that of the jurists. For the former follow the guidance of natural reason, whereas the latter are guided by civil precepts, which frequently, for the sake of some advantage, permit an act that would not otherwise be permitted.

      New explanation

      Moreover, the will is bound not only by treaties and pacts, but also by agreements tacitly indicated. For example, any person who has placed himself under the protection of another in such a way as to be in the possession and under the power of that protector, makes himself for the time being a part, as it were, of the latter, and by his silence promises clearly enough that he will devise nothing prejudicial to the welfare and sovereign status of the said protector. Thus we abhor traitors and [51′] suborned assassins and—far more intensely—poisoners.^a