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.
After the fall of the Berlin Wall in 1989, the organization became drawn into the Breakup of Yugoslavia, and conducted their first military interventions in Bosnia from 1991 to 1995 and later Yugoslavia in 1999. Politically, the organization sought better relations with former Cold War rivals, which culminated with several former Warsaw Pact states joining