The North Atlantic Treaty Organization (NATO) is an intergovernmental military alliance between 32 member states (thirty in Europe and two in North America). Founded in the aftermath of World War II, NATO was established with the signing of the North Atlantic Treaty in 1949. The organization serves as a system of collective security and deterrence, whereby its independent members agree to defend each-other from attack by any outside party. This is enshrined in Article 5 of the treaty, which states that an armed attack against the territory of one member shall be considered an attack against them all.
Throughout the Cold War, NATO's primary purpose was to deter and counter the threat posed by the Soviet Union and its satellite states, which formed the rival Warsaw Pact in 1955. Following the dissolution of the Soviet Union in 1991, the alliance adapted. NATO undertook its first military intervention in the Bosnian War (1992–1995); with a United Nations mandate, it enforced a no-fly zone and defended UN safe zones. NATO also intervened in the Kosovo War in 1999 by bombing Yugoslavia with the stated goal of ending the violence and repressive actions by the Milošević government. Article 5 was invoked for the first and only time by the United States after the September 11 attacks, leading to NATO involvement in the war in Afghanistan (2001–2014), as part of the International Security Assistance Force (ISAF). In 2011, NATO intervened in the Libyan civil war, with a UN mandate to enforce a no-fly zone and protect civilians. The alliance has since been involved in training Iraq's military and countering piracy.
Since the end of the Cold War, the alliance has accepted sixteen new members in Europe. NATO recognizes Bosnia and Herzegovina, Ukraine and Georgia as aspiring members. Russia–NATO relations had been co-operative, but became adversarial during Vladimir Putin's second presidency. Putin called NATO's eastward enlargement a threat, and opposes Ukraine joining the alliance. Following the Russian annexation of Crimea in 2014, NATO suspended co-operation and deployed a tripwire force in the east. The 2022 Russian invasion of Ukraine prompted NATO to reinforce its eastern flank and send military aid to Ukraine. The invasion caused Finland and Sweden to join the alliance. Since the invasion, Russia has been accused of waging a hybrid conflict against NATO. Since 2025, there has been internal crisis in NATO due to threats made by the US, a major contributing member, against the sovereignty of fellow members Canada and Denmark.
NATO members contribute to collective defense in two ways: by funding their own defense, and by helping to fund a shared budget for running the organization as a whole and maintaining joint facilities. Each member has committed to a target of spending at least 5% of their gross domestic product (GDP) on their own defense and security by 2035, while in 2026 the shared budget was around €5 billion split between all 32 members. As of 2026, the US spends the most overall on its defense, Norway spends the most per capita, while Poland and the Baltic states spend the most as a share of GDP. The combined military spending of NATO members makes up over half of the world total. The combined militaries of all NATO members include about 3.5 million soldiers and personnel. NATO's main headquarters are in Brussels, Belgium, while its military headquarters are near Mons, Belgium.
NATO has its roots in the Atlantic Charter, a 1941 agreement between the United States and United Kingdom. The Charter laid out a framework for international cooperation without territorial expansion after World War II. On 4 March 1947, the Treaty of Dunkirk was signed by France and the UK during the aftermath of World War II and the start of the Cold War as a Treaty of Alliance and Mutual Assistance in the event of possible attacks by Germany. In March 1948, this alliance was expanded in the Treaty of Brussels to include the Benelux countries, forming the Brussels Treaty Organization, commonly known as the Western Union. Talks for a wider military alliance, which could include North America, also began that month in the US, where their foreign policy under the Truman Doctrine set out in 1947 promoted international solidarity against actions they saw as communist aggression, such as the February 1948 coup d'état in Czechoslovakia. These talks resulted in the signature of the North Atlantic Treaty on 4 April 1949 by the member states of the Western Union and the US, Canada, Portugal, Italy, Norway, Denmark, and Iceland. Canadian diplomat Lester B. Pearson was a key author and drafter of the treaty.
The North Atlantic Treaty was largely dormant until the Korean War initiated the establishment of NATO to implement it with an integrated military structure. This included the formation of Supreme Headquarters Allied Powers Europe (SHAPE) in 1951, which adopted many of the Western Union's military structures and plans, including their agreements on standardizing equipment and agreements on stationing foreign military forces in European countries. In 1952, the post of Secretary General of NATO was established as the organization's chief civilian. That year also saw the first major NATO maritime exercises, Exercise Mainbrace and the accession of Greece and Turkey to the organization. Following the London and Paris Conferences, West Germany was permitted to rearm militarily, as they joined NATO in May 1955, which was, in turn, a major factor in the creation of the Soviet-dominated Warsaw Pact, delineating the two opposing sides of the Cold War.
The building of the Berlin Wall in 1961 marked a height in Cold War tensions, when 400,000 US troops were stationed in Europe. Doubts over the strength of the relationship between the European states and the US 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 France from NATO's military structure in 1966. In 1982, the newly democratic Spain joined the alliance.
The Revolutions of 1989 in Europe led to a strategic reevaluation of NATO's purpose, nature, tasks, and focus on the continent. In October 1990, East Germany became part of the Federal Republic of Germany and the alliance, and in November 1990, the alliance signed the Treaty on Conventional Armed Forces in Europe (CFE) in Paris with the Soviet Union. It mandated specific military reductions across the continent, which continued after the collapse of the Warsaw Pact in February 1991 and the dissolution of the Soviet Union that December, which removed the de facto main adversaries of NATO. This began a drawdown of military spending and equipment in Europe. The CFE treaty allowed signatories to remove 52,000 pieces of conventional armaments in the following sixteen years, and allowed military spending by NATO's European members to decline by 28 percent from 1990 to 2015. In 1990, several Western leaders gave assurances to Mikhail Gorbachev that NATO would not expand further east, as revealed by memoranda of private conversations. In a later interview, Gorbachev stated that an expansion of NATO was not discussed at the time, and that talks were about stationing NATO troops in former East Germany.
In the 1990s, the organization extended its activities into political and humanitarian situations that had not formerly been NATO concerns. During the breakup of Yugoslavia, the organization conducted its first military interventions in Bosnia from 1992 to 1995 and later Yugoslavia in 1999.
Politically, the organization sought better relations with the newly autonomous Central and Eastern European states, and diplomatic forums for regional cooperation between NATO and its neighbors were set up during this post-Cold War period, including the Partnership for Peace and the Mediterranean Dialogue initiative in 1994, the Euro-Atlantic Partnership Council in 1997, and the NATO–Russia Permanent Joint Council in 1998. At the 1999 Washington summit, Hungary, Poland, and the Czech Republic officially joined NATO, and the organization also issued new guidelines for membership with individualized "Membership Action Plans". These plans governed the subsequent addition of new alliance members.