The North Atlantic Treaty Organization or NATO ( /ˈneɪtoʊ/ NAY-toh; French: Organisation du traité de l'Atlantique Nord (OTAN)), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949.

Although NATO was present throughout the Cold War as the opposite to the Warsaw Pact, joint military exercises were practised but not military operation took place. This came after the Cold War when NATO's influence in a post-Cold War rose to prominence in Bosnia where NATO gradually stepped up their efforts to end the trouble. This culminated with NATO's first air campaign in 1995 which targeted the Army of Republika Srpska whose presence in Bosnia posed a danger to United Nations "safe areas" and ultimately the bombing helped to bring about the Dayton Accords.

The organization now plays a prominent role in Afghanistan after the 11 September attacks in the United States invoked Article 5 of the North Atlantic Treaty which deemed the attacks to be an external attack on all NATO members under the idea of collective defence. Aside from continuing efforts in Afghanistan NATO has participated in a wide range of roles including relief effort, counter-piracy, enforcing no-fly zones and naval blockades.

Coordinates: 50°52′34.16″N 4°25′19.24″E / 50.8761556°N 4.4220111°E / 50.8761556; 4.4220111

The North Atlantic Treaty Organization or NATO ( /ˈneɪtoʊ/ NAY-toh; French: Organisation du traité de l'Atlantique Nord (OTAN)), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949. 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. NATO's headquarters are in Brussels, Belgium, one of the 28 member states across North America and Europe, the newest of which, Albania and Croatia, joined in April 2009. An additional 22 countries participate in NATO's Partnership for Peace, with 15 other countries involved in institutionalized dialogue programs. The combined military spending of all NATO members constitutes over 70% of the world's defence spending.

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 course of the Cold War led to a rivalry with nations of the Warsaw Pact, which formed in 1955. The first NATO Secretary General, Lord Ismay, stated in 1949 that the organization's goal was "to keep the Russians out, the Americans in, and the Germans 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 defence against a prospective Soviet invasion—doubts that led to the development of the independent French nuclear deterrent and the withdrawal of the French from NATO's military structure in 1966.