Collective security can be understood as 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. 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. While collective security is an idea with a long history, its implementation in practice has proved problematic. Several prerequisites have to be met for it to have a chance of working. It is the theory or practice of states pledging to defend one another in order to deter aggression or to target a transgressor if international order has been breached.

Collective security is also referred to by the phrase "an attack on one is an attack on all". However, usage of this phrase also frequently refers to Article 5 of the North Atlantic Treaty, the collective security provision in NATO's charter.