The North Atlantic Treaty Organization (NATO), also called the North Atlantic Alliance, is an intergovernmental military alliance between 29 North American and European countries based on the North Atlantic Treaty that was signed on 4 April 1949.NATO constitutes a system of collective defence whereby its independent member states agree to mutual defence in response to an attack by any external party. The combined military spending of all NATO members constitutes over 70% of the global defence spending. NATO Headquarters are located in Haren, Brussels, Belgium, while the headquarters of Allied Command Operations is near Mons, Belgium. Its other official name is the French equivalent, l’Organisation du Traité de l’Atlantique Nord (OTAN) (English and French being the two official languages of the organisation).