This chapter deals with the lack of coherency between strategic reality—which uses deterrence as an essential strategic tool—and the prevailing law. Deterrence is a tool for enforcing compliance with the law; it promotes the containment of potential conflicts. It is pivotal in strategic thinking and, in many cases, an essential component of the national-defense strategy of law-abiding states. But although deterrence is central to the management of global security, in current international law deterrence considerations are perceived with suspicion and mistrust. It is perceived as an unlawful punitive measure. The lack of consensus on lawful deterrence, however, might create a vacuum that invites aggressors and transgressors. This chapter offers normative suggestions for introducing defensive deterrence and overcoming the practical problem of putting it into practice within the current contours of the law, by using the military professionalism criterion.