NATO is an intergovernmental military alliance in the defense sector between 32 member states, including 30 European and two North American, established in the aftermath of the Second World War. The founding treaty of NATO, the Atlantic Pact, was signed in Washington on 4 April 1949, and entered into force on 24 August of the same year. NATO's main headquarters is located in Brussels, Belgium, while the military headquarters is located near Mons, also in Belgium.

The alliance has the NATO Response Force and the combined armed forces of all its members comprising a total of approximately 3.5 million soldiers and various personnel. Members agreed to meet or maintain a defense spending target of at least 2% of their GDP in 2024. In 2024, NATO recognizes Bosnia and Herzegovina, Georgia and Ukraine as aspiring members. The expansion of the alliance has led to tensions with third countries such as Russia, one of nineteen countries participating in NATO's Partnership for Peace program. Nineteen other countries are involved in institutionalized dialogue programs with the alliance.