After a long history of moral condemnation of mercenarism, political philosophers have recently shown a friendlier face towards the employment by states of private parties in war conflicts. I argue that even if traditional, motivational arguments may fail to provide compelling reasons to condemn mercenarism, recent defenses ignore the fact that mercenarism, at least in its current form, consists in a system of morally problematic, and possibly invalid, promises between states and private actors. More precisely, I advance the following claims: (1) either the mercenary’s offer of combat military services to the state is invalid (because it entails the alienation of certain rights that individuals lack the moral power to alienate through simple market transactions) or, if valid, (because limited to waiving those rights rather than alienating them), then, states are under a duty not to accept that offer, for doing so would compromise the purpose of war. Further, (2) the mercenary’s acceptance of the state’s offer to provide benefits in exchange of combat services is itself seriously problematic for this offer often confers to the mercenary a kind of wide decisional authority that the mercenary either lacks the moral power to accept or, in any case, ought to reject. A possible solution to this problem would consist in a thorough reform of international rules. However, the changes that are empirically necessary to legitimize the privatization of war are also likely to be self-defeating: they defeat those reasons that arguably make the privatization of war appealing in the first place.