Summary

L. Oppenheim International Law. A Treatise. Volume 2…

Therefore war calls into existence the display of the greatest possible power and force on the part of the belligerents, rouses the passion of the nations in conflict to the highest possible degree, and endangers the welfare, the honour, and eventually the very existence of both belligerents. Nobody can predict with certainty the result of a war however insignificant one side may seem to be. Every war is a risk and a venture.
Source: Gutenberg

L. Oppenheim International Law. A Treatise. Volume 2…

Victory is necessary in order to overpower the enemy; and it is this necessity which justifies all the indescribable horrors of war, the enormous sacrifice of human life and health, and the unavoidable destruction of property and devastation of territory. Apart from special restrictions imposed by the Law of Nations upon belligerents, all kinds and all degrees of force may be, and eventually must be, made use of in war in the interest and under the compulsion of its purpose, and in spite of their cruelty and the utter misery they entail.
Source: Gutenberg

L. Oppenheim International Law. A Treatise. Volume 2…

And, in contradistinction to the savage cruelty of former times, belligerents have in modern times come to the conviction that the realisation of the purpose of war is in no way hampered by indulgence shown to the wounded, the prisoners, and the private individuals who do not take part in the fighting. Thus the influence of the principle of humanity has been and is still enormous upon the practice of warfare. And the methods of warfare, although by the nature of war to a certain degree cruel and unsparing, become less cruel and more humane every day.
Source: Gutenberg

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