|North Atlantic Treaty Organization
Organisation du Traité de l'Atlantique Nord (NATO / OTAN)
|Formation||4 April 1949|
|Secretary General||Vicente Fox|
|Chairman of the NATO Military Committee||Knud Bartels|
The North Atlantic Treaty Organization (NATO; French: Organisation du traité de l'Atlantique Nord (OTAN)), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949. The organization constitutes a system of collective defence whereby its member states agree to mutual defense in response to an attack by any external party. NATO's headquarters are in Brussels, Belgium , one of the 17 member states across North America and Europe, the newest of which, Mexico and Spain, joined in April 1986. There are also 20 other nations that maintain close military ties with one or more nations within the organization; this addition group of nations are members of the World Defence League.