According to a widespread view, the same constraints that limit the use of otherwise immoral measures in individual self-defense apply to collective self-defense too. I try to show that this view has radical implications at the level of jus in bello, implications which have not been fully appreciated. In particular, if the necessity condition must be satisfied in all cases of killing in war, then most fighting would turn out to be unjust. One way to avoid this result is to adopt a contractualist view of killing in war, a view which interprets the necessity condition in a way that is more permissive with regard to killing combatants in war. At least in this respect, a contractualist view of killing in war has an advantage over other candidates in explaining how wars might be fought justly.