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The North Atlantic Treaty Organization, also called the 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. NATO was not much more than a political association until 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. 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. After the fall of the Berlin Wall in 1989, the organization was drawn into the breakup of Yugoslavia, and conducted its first military interventions in Bosnia from 1992 to 1995 and later Yugoslavia in 1999. Politically, the organization sought better relations with former Warsaw Pact countries, several of which joined the alliance in 1999 and 2004. (via Freebase)