It is sometimes said that cruel yet short wars are better for humanity than restrained yet lengthy ones. The idea finds sympathy among Francis Lieber and his Prussian contemporaries, as well as some modern writers who back selective non-compliance with international humanitarian law (IHL) on act-utilitarian grounds. This article refutes three underlying claims and reaffirms that IHL progressively narrows room for crude interest-balancing by its duty-bearers. First, it is claimed that toughening wars quickens them, whereas moderating wars prolongs them. This empirical claim overlooks how actions of the party resorting to brutality – “brutaliser” for short – interact with the intention of its adversary. Although the brutaliser clearly controls the amount of violence it chooses to inflict on its opponent, it does not control the opponent’s will to resist and, consequently, the length of the war it fights. History abounds with instances where adding cruelty stiffens the enemy’s resolve rather than accelerates surrender. Second, it is claimed that ruthless but swift wars lessen net inhumanity. On this act-utilitarian view, it is normatively superior to hasten wars through barbarity than to lengthen them through moderation. It is therefore the brutaliser’s responsibility to toughen fighting and the brutalised party’s responsibility to refrain from resisting the brutaliser. Problematically, the brutaliser usurps authority by imposing its own utilitarian considerations upon the brutalised party. Moreover, the brutaliser blames its disobliging adversary for the extra bloodshed to which it resorts in the name of maximum utility. Third, it is claimed that IHL does or should permit nonconformity when nonconformity stands reasonable chances of increasing net humanity. This position is inconsistent with IHL’s functions. IHL does aim to reduce net wartime harm. It would be a mistake to assume, however, that utilitarian ends necessarily justify, let alone require, utilitarian means. When IHL enacts unqualified rules, it predetermines their conformity or nonconformity through processes that are distinctly not act-utilitarian. Nowhere in these processes do lesser-evil justifications naturally belong.