The Philosophy of Law: Public Right, § 57: Right during War
The determination of what constitutes Right in War, is the most difficult problem of the Right of Nations and International Law. It is very difficult even to form a conception of such a Right, or to think of any Law in this lawless state without falling into a contradiction. Inter arma silent leges. It must then be just the right to carry on War according to such principles as render it always still possible to pass out of that natural condition of the states in their external relations to each other, and to enter into a condition of Right.
No war of independent States against each other, can rightly be a war of Punishment (bellum punitivum). For punishment is only in place under the relation of a Superior (imperantis) to a Subject (subditum); and this is not the relation of the States to one another. Neither can an international war be ‘a war of Extermination’ (bellum internicinum), nor even ‘a war of Subjugation’ (bellum subfugatorium); for this would issue in the moral extinction of a State by its people being either fused into one mass with the conquering State, or being reduced to slavery. Not that this necessary means of attaining to a condition of peace is itself contradictory to the right of a State; but because the idea of the Right of Nations includes merely the conception of an antagonism that is in accordance with principles of external freedom, in order that the State may maintain what is properly its own, but not that it may acquire a condition which, from the aggrandizement of its power, might become threatening to other States.
Defensive measures and means of all kinds are allow able to a State that is forced to war, except such as by their use would make the Subjects using them unfit to be citizens; for the State would thus make itself unfit to be regarded as a person capable of participating in equal rights in the international relations according to the Right of Nations. Among these forbidden means are to be reckoned the appointment of Subjects to act as spies, or engaging Subjects or even strangers to act as assassins, or poisoners (in which class might well be included the so-called sharpshooters who lurk in ambush for individuals), or even employing agents to spread false news. In a word, it is forbidden to use any such malignant and perfidious means as would destroy the confidence which would be requisite to establish a lasting peace thereafter.
It is permissible in war to impose exactions and contributions upon a conquered enemy; but it is not legitimate to plunder the people in the way of forcibly depriving individuals of their property. For this would be robbery, seeing it was not the conquered people but the State under whose government they were placed that carried on the war by means of them. All exactions should be raised by regular Requisition, and receipts ought to be given for them, in order that when peace is restored the burden imposed on the country or the province may be proportionately borne.