The North Atlantic Treaty Organization (NATO; , )); ), also called "the (North) Atlantic Alliance", is a military alliance established by the signing of the North Atlantic Treaty on 4 April 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 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 from 1966.

After the fall of the Berlin Wall in 1989, the organization became drawn into the Balkans while building better links with former potential enemies to the east, which culminated with several former Warsaw Pact states joining the alliance in 1999 and 2004. On April 1, 2009, membership was enlarged to 28 with the entrance of Albania and Croatia. Since the September 11, 2001 terrorist attacks, NATO has attempted to refocus itself to new challenges and has deployed troops to Afghanistan as well as trainers to Iraq.

The Berlin Plus agreement is a comprehensive package of agreements made between NATO and the EU on 16 December 2002. With this agreement the EU was given the possibility to use NATO assets in case it wanted to act independently in an international crisis, on the condition that NATO itself did not want to act – the so-called "right of first refusal". Only if NATO refused to act would the EU have the option to act. The combined military spending of all NATO members constitutes over 70% of the world's defence spending, with the United States alone accounting for about half the total military spending of the world and the United Kingdom and France accounting for a further 10%.

Beginnings

thumb|The North Atlantic Treaty was signed in Washington, D.C. on 4 April 1949 and was ratified by the United States that August. The Treaty of Brussels, signed on 17 March 1948 by Belgium, the Netherlands, Luxembourg, France and the United Kingdom is considered the precursor to the NATO agreement. The treaty and the Soviet Berlin Blockade led to the creation of the Western European Union's Defense Organization in September 1948. However, participation of the United States was thought necessary in order to counter the military power of the USSR, and therefore talks for a new military alliance began almost immediately.