NATO was little more than a political association until the Korean War galvanized the organization's member states, and an integrated military structure was built up under the direction of two US Supreme Commanders. The course of the Cold War led to a rivalry with nations of the Warsaw Pact, that 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 defense against a prospective Soviet invasion—doubts that led to the development of the independent French nuclear deterrent and the withdrawal of France from NATO's military structure in 1966 for 30 years. After the fall of the Berlin Wall in Germany in 1989, the organization conducted its first military interventions in Bosnia from 1992 to 1995 and later Yugoslavia in 1999 during the breakup of Yugoslavia. Politically, the organization sought better relations with former Warsaw Pact countries, several of which joined the alliance in 1999 and 2004.
Article 5 of the North Atlantic treaty, requiring member states to come to the aid of any member state subject to an armed attack, was invoked for the first and only time after the September 11 attacks, after which troops were deployed to Afghanistan under the NATO-led ISAF. The organization has operated a range of additional roles since then, including sending trainers to Iraq, assisting in counter-piracy operations and in 2011 enforcing a no-fly zone over Libya in accordance with UN Security Council Resolution 1973. The less potent Article 4, which merely invokes consultation among NATO members, has been invoked five times: by Turkey in 2003 over the Iraq War; twice in 2012 by Turkey over the Syrian Civil War, after the downing of an unarmed Turkish F-4 reconnaissance jet, and after a mortar was fired at Turkey from Syria; in 2014 by Poland, following the Russian intervention in Crimea; and again by Turkey in 2015 after threats by Islamic State of Iraq and the Levant to its territorial integrity.
Since its founding, the admission of new member states has increased the alliance from the original 12 countries to 29. The most recent member state to be added to NATO is Montenegro on 5 June 2017. NATO currently recognizes Bosnia and Herzegovina, Georgia, Macedonia and Ukraine as aspiring members. An additional 21 countries participate in NATO's Partnership for Peace program, with 15 other countries involved in institutionalized dialogue programs. The combined military spending of all NATO members constitutes over 70% of the global total. Members' defense spending is supposed to amount to at least 2% of GDP by 2024.
The Treaty of Brussels was a mutual defence treaty against the Soviet threat at the start of the Cold War. It was signed on 17 March 1948 by Belgium, the Netherlands, Luxembourg, France, and the United Kingdom. It was the precursor to NATO. The Soviet threat became immediate with the Berlin Blockade in 1948, leading to the creation of a multinational defence organization, the Western Union Defence Organisation, in September 1948. However, the parties were too weak militarily to counter the Soviet Armed Forces. In addition, the 1948 Czechoslovak coup d'état by the Communists had overthrown a democratic government and British Foreign Minister Ernest Bevin reiterated that the best way to prevent another Czechoslovakia was to evolve a joint Western military strategy. He got a receptive hearing in the United States, especially considering American anxiety over Italy (and the Italian Communist Party).
In 1948, European leaders met with US defence, military and diplomatic officials at the Pentagon, under US Secretary of State George C. Marshall's orders, exploring a framework for a new and unprecedented association. Talks for a new military alliance resulted in the North Atlantic Treaty, which was signed by US President Harry S. Truman in Washington on 4 April 1949. It included the five Treaty of Brussels states plus the United States, Canada, Portugal, Italy, Norway, Denmark and Iceland. 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". Popular support for the Treaty was not unanimous, and some Icelanders participated in a pro-neutrality, anti-membership riot in March 1949. The creation of NATO can be seen as the primary institutional consequence of a school of thought called Atlanticism which stressed the importance of trans-Atlantic cooperation.