This paper sketches a conventionalist account of the rules of war, called the 'Quid Pro Quo Account'. Conventionalist accounts of war arise from the intuitive inadequacy of accounts of war which reduce the rules of engagement to the permissions and prohibitions supported by theories of individual liability. The Quid Pro Quo Account contends that combatants lose their non-liability to being killed through a purely conventional appointment to the role of ‘combatant’ whose deep aim is to protect the bulk of the civilian population from danger, and in return for which these combatants can expect two significant quid pro quos: first, that the combatants be armed, and second, that the civilian killings which combatants are responsible for in carefully specified circumstances do not amount to homicide. This immunity-conferring quid pro quo emerges, in effect, as a moral coordination problem faced by two opposing armies. Given the asymmetrical conflict enjoined by terrorist actors, it emerges that terrorists cannot collect this sort of immunity, from which it follows that their killings of civilians will remain merely homicidal. The Quid Pro Quo Account is also defended against some more general objections to conventionalism.