The North Atlantic Treaty Organization or NATO also called the North Atlantic Alliance, is an intergovernmental military alliancebased on the North Atlantic Treaty which was signed on 4 April 1949. The NATO headquarters are in Brussels and 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.

For its first few years, NATO was not much more than a political association. However, the Korean War galvanized the member states, and an integrated military structure was built up under the direction of two U.S. supreme commanders.

The first NATO Secretary GeneralLord Ismay, famously stated the organization's goal was "to keep the Russians out, the Americans in, and the Germans down"

NATO has added new members seven times since first forming in 1949 (the last two in 2009). NATO comprises 28 members: Albania, Belgium, Bulgaria, Canada, Croatia, Czech Republic,Denmark, Estonia, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania,Luxembourg, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Turkey, the United Kingdom, and the United States.

Each of the 28 members have a delegation or mission to NATO's headquarters in Brussels.