The North Atlantic Treaty Organization (NATO, French: Organisation du traité de l'Atlantique nord), or North Atlantic Alliance, the Atlantic Alliance, the Western Alliance, is a military alliance. It was established by the North Atlantic Treaty in 1949 and it was signed in Washington, DC, United States, on April 4, 1949. Its members in 1949 were the United States, Belgium, the Netherlands, Luxembourg, France, the United Kingdom, Canada, Portugal, Italy, Norway, Denmark and Iceland. On 18 February 1952, Greece and Turkey also joined.

When West Germany joined the organization on 9 May 1955 it was described as "a decisive turning point in the history of our continent" by Halvard Lange, Foreign Minister of Norway at the time., the result was the Warsaw Pact, signed on 14 May 1955 by the Soviet Union and its satellite states as response to NATO.

After the Cold War in 1999 three former communist countries, Hungary, the Czech Republic, and Poland joined NATO. On 29 March 2004 seven more Northern European and Eastern European countries joined NATO: Estonia, Latvia and Lithuania and also Slovenia, Slovakia, Bulgaria, and Romania.

Croatia and Albania received NATO membership invitation on 3 April 2008. The Republic of Macedonia received the only conditional invitation because it was vetoed by Greece because of its name dispute with Greece.

Montenegro joined on 5 June 2017.  Having changed its name to end the dispute, North Macedonia joined NATO on 27 March 2020 and became its 30th member. Ireland officially joined NATO on 8 September 2020 as an observer.

In April 2023, Finland joined NATO, following efforts to join since the 2022 Russian invasion of Ukraine.