This article introduces three ways in which a state at war can attempt to accommodate the often contradictory demands of military necessity and humanitarianism – three ‘logics’ of waging war. The logics of sufficiency, efficiency and moral liability differently distribute the harm and destruction that waging war inevitably causes. International law demands belligerents follow the logic of sufficiency. Contemporary strategic imperatives, to the contrary, put a premium on waging war efficiently. Cross-culturally shared expectations of proper state conduct, however, mean killing in war ought to fit the logic of moral liability. The latter proves entirely impracticable. Hence, a belligerent faces a choice: (i) renounce the right and capacity to use large-scale collective force in order to meet public expectations of morally appropriate state conduct (logic of liability); (ii) defy those expectations as well as international law and follow strategic imperatives (logic of efficiency) and (iii) follow international law (logic of sufficiency), which is inefficient and will be perceived as illegitimate. This is the 21st-century belligerent’s trilemma.