Security is fundamental for every nation-state for its survival since ancient times. Today's security theory is required to construct a new concept for the 21st century against the background of the transition from the concept of national defense (Defense) to the multidimensional concept of security (Security). It is necessary to examine from various perspectives the implications for international security of increasingly complex post-Cold War regional conflicts, ethnic conflicts, civil wars among failed states, the rise of asymmetric actors after the 9/11 terrorist attacks, the rise of emerging powers led by China (power shift), and the impact on the security of the emergence of new technologies. In this lecture, we will deepen our discussion from the following perspectives: 1) the concept of security and policy systems, 2) spectrum analysis of international conflicts, 3) case studies of symmetrical and asymmetrical conflicts, 4) security policies at various stages of prevention, restraint, deterrence, conflict intervention, peacekeeping, and peacebuilding, 5) regional security architecture in Europe and Asia. The discussion will be deepened from the following perspectives.