The North Atlantic Treaty Organization (NATO, ; French: Organisation du traité de l'Atlantique nord, OTAN), also called the North Atlantic Alliance, is an intergovernmental military alliance between 31 member states – 29 European and two North American. Established in the aftermath of World War II, the organization implemented the North Atlantic Treaty, signed in Washington, D.C., on 4 April 1949. NATO is a collective security system: its independent member states agree to defend each other against attacks by third parties. During the Cold War, NATO operated as a check on the threat posed by the Soviet Union. The alliance remained in place after the dissolution of the Soviet Union and has been involved in military operations in the Balkans, the Middle East, South Asia, and Africa. The organization's motto is animus in consulendo liber (Latin for "a mind unfettered in deliberation").

NATO's main headquarters are located in Brussels, Belgium, while NATO's military headquarters are near Mons, Belgium. The alliance has targeted its NATO Response Force deployments in Eastern Europe, and the combined militaries of all NATO members include around 3.5 million soldiers and personnel. Their combined military spending as of 2020 constituted over 57 percent of the global nominal total. Moreover, members have agreed to reach or maintain the target defence spending of at least two percent of their GDP by 2024.

NATO formed with twelve founding members and has added new members nine times, most recently when Finland joined the alliance on 4 April 2023, exactly 74 years after NATO's formation. Following the acceptance of its application for membership in June 2022, Sweden is anticipated to become the 32nd member, with its Accession Protocol to the North Atlantic Treaty now in the process of being ratified by the existing members. In addition, NATO currently recognizes Bosnia and Herzegovina, Georgia, and Ukraine as aspiring members. Enlargement has led to tensions with non-member Russia, one of the twenty additional countries participating in NATO's Partnership for Peace programme. Another nineteen countries are involved in institutionalized dialogue programmes with NATO.