The North Atlantic Treaty Organization, also called the North Atlantic Alliance, is an intergovernmental military alliance between 29 North American and European countries. The organization implemented the North Atlantic Treaty that was signed on 4 April 1949. NATO constituents a system of collective defense wheree its independent member states agree to mutual defense in response to an attack by any external party. NATO's Headquarters are located in Haren, Brussels, Belgium, while the headquarters of Allied Command Operations is near Mons, Belgium.