But such was not the public opinion or practice of enlightened Europe at the time they wrote. Grotius had long before, even in opposition to his own authorities, but influenced by religion and humanity, mentioned that many things were not fit and commendable, though they might be strictly lawful. He held that the Law of Nations prohibited the use of poisoned arms, the employment of assassins, violence to women or the dead, or making slaves of prisoners. Montesquieu followed in the same humane spirit. He writes, that the civilians said, "That the law of nations, to prevent prisoners being put to death, has allowed them to be made slaves.... The reasons of the civilians are all false. It is false, that killing in war is lawful, unless in case of absolute necessity; but when a man has made another his slave, he cannot be said to be under a necessity of taking away his life, since he actually did not take it away. War gives no other right over prisoners than to disable them from doing any further harm, by securing their persons. All nations concur in detesting the murdering of prisoners in cold blood."[79] Thus, it is now the established Law of Nations, that necessity is the measure of violence in war, and humanity, its tempering spirit; or, as it has been otherwise enunciated, the rights of war are to be measured by the objects of the war. Although we have a right to kill our enemies in war; it is only when we find gentler methods insufficient to conquer their resistance and bring them to terms, that we have a right to put them to death.[80] Under the name of enemies are comprehended not only the first author of the war, but also those who join him and support his cause. [Sidenote: Cartel] Out of these enlightened views of war has sprung the System of Cartels for the exchange of prisoners. These exchanges are generally regulated by special convention between the hostile states. Prisoners are sometimes permitted to return home, upon condition not to serve again during the war, or until duly exchanged. Officers are frequently released upon their parole, on the same condition; and to carry more effectually into operation the arrangements necessary for these purposes, commissaries are permitted to reside in the respective hostile states. Subject to the principle of non-resistance, there are several classes of persons that are generally considered exempt from the operations of war, beyond the effects of unavoidable accident. "All the members of the enemy's state," says Wheaton,