NATO’s first phase of large-scale military organization wasn’t directly related to any threat against its 12 member countries. Instead, it stemmed from a proxy war with the Soviet Union in which the United States and other NATO members felt entitled to intervene.

After WWII, the Allied nations divided the Korean peninsula along the 38th Parallel, leading to the development of a Soviet-aligned government in the north and an American-aligned government in the south. In 1950, North Korea invaded South Korea, sparking the Korea War. The United States and other NATO members individually sent troops to fight North Korea. Meanwhile, NATO began to assemble and organize its military infrastructure under the assumption that it would need to fight against future attacks from the Soviet Union and its allies.

NATO established a military headquarters known as Supreme Headquarters Allied Powers Europe (SHAPE), appointing the Allied forces’ Supreme Commander Dwight D. Eisenhower to lead it. In addition, NATO established a long-term defense plan and conducted its first large-scale naval exercise. By the time the Korean War formally ended in 1953, NATO had established a significant international military presence.

The original 12 countries that formed NATO in 1949 included Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States. In 1952, NATO gained two new members, Greece and Turkey. Then, in 1955, NATO allowed West Germany to join its ranks.

After WWII, the Allies divided Nazi Germany and began a disarmament program to prevent future German military aggression. West Germany allied itself with the United States and many of the other NATO countries, while East Germany allied itself with the Soviet Union.

However, in 1955, West Germany joined NATO and began to rearm itself. The Soviet Union was not happy.

Less than two weeks after West Germany officially joined NATO, the Soviets and seven other countries in eastern Europe signed the Warsaw Pact. Like the 1949 North Atlantic Treaty that established NATO, the Warsaw Pact called on all its signatories to defend each other in case of attack. Along with the Soviet Union, the pact included Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland and Romania.

NATO is a political as well as a military organization. However, in 1966, one of NATO’s founding members—France—decided to withdraw from its military commitments to NATO.

The French president who made this decision was Charles de Gaulle, the famous brigadier general who had helped liberate Nazi-occupied France and establish an interim government immediately after the Germans retreated. Even though France had played an important role during NATO’s early years, De Gaulle had grown unhappy with France’s position in the organization and clashed with other member states. In addition, he wanted to establish French military independence.

France didn’t rejoin NATO until 2009, under President Nicolas Sarkozy. By that time, a lot had changed: The Vietnam War and the Cold War had ended, 9/11 had happened and the U.S. was several years into its wars in Afghanistan and Iraq.

NATO established a robust military presence during the Cold War, with Article 5 of the North Atlantic Treaty calling for collective defense. However, NATO never officially invoked the article during the Cold War. In fact, the first time NATO ever invoked Article 5 was in the wake of the September 11 attacks on the United States in 2001. That October, NATO launched its first anti-terror operation, in which multiple NATO countries assisted in patrolling the airspace over the United States.

By this time, NATO had gained four more members: Spain joined in 1982, and the Czech Republic, Hungary and Poland joined in 1999. Significantly, the last three states were former members of the Warsaw Pact, which ended in 1991.

The reunification of East and West Germany and the dissolution of the Soviet Union signaled the end of the Cold War. Over the next three decades, former members of the Warsaw Pact began to pile into NATO.

East Germany became the first former Soviet ally to technically join when it reunited with West Germany, which was already a member. In addition to the Czech Republic, Hungary and Poland in 1999, NATO gained the former Warsaw states of Bulgaria, Romania and Slovakia (previously part of Czechoslovakia with the Czech Republic) in 2004. Albania, which withdrew from the Warsaw Pact in 1968, joined NATO in 2009.

The Baltic states of Estonia, Latvia and Lithuania—which were part of the Soviet Union—also joined NATO in 2004. The six nations that round out NATO’s 32 members are Slovenia (joined 2004), Croatia (2009), Montenegro (2017), North Macedonia (2020), Finland (2023) and Sweden (2024).