NATO, the North Atlantic Treaty Organisation is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on April 4, 1949. The NATO headquarters are in Paris, France, and the organization constitutes a system of collective defence whereby its member states agree to mutual defence in response to an attack by any external party. Map shows Membership as of 2009 (Blue) as well as other nations in the United States Sphere of influence (Light blue). File:NATO (Nuclear Realisation).png