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A more accurate definition for security study purposes can define Collective Defence as "an agreement between two or more states to work together on mutual security issues." .
A "Region" for our purposes can be simply defined "as a set of states, which are located in geographical proximity to one another." However, this does not imply that the state actors within a region have to align themselves with their neighbours to form a Collective Defence agreement. .
The ANZUS treat provides us with an example of this, with Australia and New Zealand located in a geographical region and the United States situated outside this region, but the treaty still provides all concerned with different elements of regional security.
Collective Defence differs from other forms of security measures in a number of ways. States within a region usually enter into an agreement to co-operate with each other on security arrangements in order to safeguard their own state interests against a mutual or perceived threat to the states within the agreement. By operating under this mechanism of security, states band their military resources together and act in concert in any military or non-military involvement. This has the combined affect of elevating their power status in the international arena, improve their state security and may gain political advantage within the region, comparative to external states not within the scope of the arrangement.
These agreements/arrangements can be either formal or informal agreements/arrangements. Formal agreements may involve the states concerned signing treaties whereby signatories agree to provide military force if anyone of the signatories come under attack. An attack on one signatory is considered an attack on all signatories. The most well known example of a formal agreement under a Collective Defence arrangement would be the North Atlantic Treaty Organisation (NATO).
