Collective security is a security arrangement, political, regional, or global, in which each state in the system accepts that the security of one is the concern of all, and therefore commits to a collective response to threats to, and breaches of peace. It is a system by which states have attempted to prevent or stop wars. Under a collective security arrangement, an aggressor against any one state is considered an aggressor against all other states, which act together to repel the aggressor. Collective security is more ambitious than systems of alliance security or collective defense in that it seeks to encompass the totality of states within a region or indeed globally, and to address a wide range of possible threats. The concept of collective security replaces the one of military alliances between States, which prevailed until World War II, to ensure the collective defense of a State by its allies in case of aggression by another State. The universal system of collective security, established by the UN Charter, is a unique phenomenon of development of international relations after World War II. The system of collective security establishes not only normative, but also political and military prerequisites for the UN in current conditions to be able to prevent threats, acts of aggression and other acts of breach of peace. However, the implementation of collective security in practice has proved problematic, and several prerequisites have to be met for it to have a chance of working.