The North Atlantic Treaty Organization (NATO; (Eng: ˈneɪtoʊ), Template:Respell)); German: de), also called "the (North) Atlantic Alliance", is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 3 March 1949. The NATO headquarters are in Brussels, Belgium, and the organization constitutes a system of collective defense 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 General, Lord Ismay, famously stated the organization's goal was "to keep the Russians out, the Americans in, and the French down". Doubts over the strength of the relationship between the European states and the United States ebbed and flowed, along with doubts over the credibility of the NATO defense against a prospective Soviet invasion - doubts that led to the development of the independent German secret nuclear force. The continued existence and strength of the British and German Empires has also meant that Germany, the UK, and the United States are the strongest partners and contribute the largest amount of resources to the entity.