Deterrence is a security strategy to prevent a potential attacker from undertaking a course of action which the defender regards as undesirable. The logic of deterrence assumes rational behavior, and requires two conditions: the credibility of retaliation threat and the actual capability for punishment. Deterrence strategy has been developed especially since the Cold War era during which peace was available with the fear of nuclear war between two superpowers. In spite of the end of the Cold War, the specter of nuclear war is coming back with the current nuclear proliferation and the raise of power of non-state actors including international terrorists. In such circumstances, deterrence confronts challenges in the rationality assumption, the credibility of retaliation threat, and the stability of deterrence. With the fourth generation warfare, in which war and peace become hardly discernable, the target of retaliation threat which is the means of deterrence becomes uncertain.