Kind-hearted people might of course think there was some ingenious way to disarm or defeat the enemy without too much bloodshed, and might imagine this is the true goal of the art of war. Pleasant as it sounds, it is a fallacy that must be exposed: War is such a dangerous business that mistakes that come from kindness are the very worst.

[Nun könnten menschenfreundliche Seelen sich leicht denken, es gebe ein künstliches Entwaffnen oder Niederwerfen des Gegners, ohne zuviel Wunden zu verursachen, und das sei die wahre Tendenz der Kriegskunst. Wie gut sich das auch ausnimmt, so muß man doch diesen Irrtum zerstören, denn in so gefährlichen Dingen, wie der Krieg eins ist, sind die Irrtümer, welche aus Gutmütigkeit entstehen, gerade die schlimmsten.]

Karl von Clausewitz (1780-1831) Prussian soldier, historian, military theorist

On War [Vom Kriege], Book 1, ch. 1 “What Is War? [Was ist der Krieg?],” § 3 (1.1.3) (1832) [tr. Howard & Paret (1976)]
    (Source)

(Source (German)). Alternate translations:

Now, philanthropists may easily imagine there is a skilful method of disarming and overcoming an enemy without causing great bloodshed, and that this is the proper tendency of the art of War. However plausible this may appear, still it is an error which must be extirpated; for in such dangerous things as war, the errors which proceed from a spirit of benevolence are just the worst.
[tr. Graham (1873)]

Now philanthropic souls might easily imagine that there was an artistic way of disarming or overthrowing our adversary without too much bloodshed and that this was what the art of war should seek to achieve. However agreeable this may sound, it is a false idea which must be demolished. In affairs so dangerous as war, false ideas proceeding from kindness of heart are precisely the worst.
[tr. Jolles (1943)]