NATO: The World’s Largest Alliance
What is NATO? In this timeline, trace NATO’s history and learn how the organization’s mission has evolved over seventy years.
Teaching Resources—Conflict: Extension (including lesson plan with slides)
As World War II ended and the relationship between the Soviet Union and other Allied powers deteriorated, the United States, Canada, and a group of ten European countries organized themselves for a new geopolitical reality. In 1949, they created the North Atlantic Treaty Organization (NATO), an alliance with the aim of preserving a free, integrated, and democratic Europe, particularly against the Soviet Union.
In international relations, an alliance is a group of countries that come together to promote their common security interests. In many cases, alliances promise military support if one member is threatened, a formal commitment known as mutual security. The core of NATO’s strength comes from Article 5 of its founding treaty—a commitment that an attack on one member country will be considered an attack on the entire alliance.
All kinds of countries benefit from alliances. For smaller, weaker countries that lack resources to mount a proper defense, joining an alliance can be the only realistic path to maintaining security and deterring potential foes. Larger or stronger countries use alliances to increase their military might and, sometimes, their influence over smaller countries, while also discouraging nuclear proliferation among smaller countries.
For over seven decades, NATO has endured, even as the common foe that it was organized against—the Soviet Union—disappeared. It successfully protected its members against Soviet aggression; no NATO member was ever attacked by the Soviet Union. Today, the alliance consists of thirty-one member countries and is an essential reason that Europe has remained mostly conflict-free since the end of World War II. But it has also recently taken on faraway missions in places like the Middle East to prevent humanitarian suffering, with mixed results, and come under increased scrutiny due to many of its European members not spending enough on their defense. These challenges come as many NATO countries consider Russia a renewed threat to Europe’s security following its aggression in Ukraine.
This timeline explores NATO’s history—from its origins as a bulwark against the Soviet Union to its present-day operations far from Europe’s shores—and examines the effects of its expanding membership, evolving mission, and recent funding concerns, all of which point toward an uncertain future.
As World War II came to a close, the alliance between the Soviet Union and Western powers deteriorated. Europe needed a plan for maintaining long-term peace. Under terms outlined at the Yalta and Potsdam Conferences, Soviet Premier Joseph Stalin was given temporary control of Poland and a portion of Germany. However, it quickly became clear that the Soviet Union had greater ambitions, as it sponsored communist-led coups in Czechoslovakia and surrounding east European territories. Fearful of further Soviet encroachment on Western Europe, these countries discussed forming an organization capable of providing mutual security.
The Soviet Union’s alarming expansion and failure to comply with the terms of the Potsdam and Yalta Conferences led to the creation of NATO as a means to preserve a unified, democratic Europe. The defensive alliance was formed in 1949 and consisted of twelve members: the United States, Canada, and ten European countries. NATO’s founding members structured the organization so that all decisions would be made through consensus. Despite the alliance’s egalitarian structure, the United States’ leading role was clear from the start; a supreme allied commander Europe (SACEUR), a position that an American has always held, leads NATO’s military operations.
NATO’s founding members believed that expanding the alliance was crucial to ensuring peace in Europe. They offered membership to Greece and Turkey in 1952 and West Germany in 1955, the latter an especially consequential decision given Germany’s history of provoking conflict in Europe. With its entry into NATO, West Germany became more integrated with Western Europe and committed to never manufacturing weapons of mass destruction. In the words of NATO’s first secretary-general, the purpose of NATO was to “keep the Soviet Union out, the Americans in, and the Germans down.”
West Germany’s entrance into NATO prompted the Soviet Union to create its own alliance: the Warsaw Pact. Although the Soviet Union already had bilateral defense agreements with most of its communist neighbors, the Warsaw Pact formalized these relations into a bloc that could act in a unified manner. By focusing on integrating the militaries of Warsaw Pact countries, the Soviet Union could control smaller east European countries more effectively, shielding them from NATO influence. The Soviet Union went to great lengths to retain these allies: when Hungarians demonstrated in 1956, calling for greater political and economic freedoms and the country’s withdrawal from the Warsaw Pact, the Soviet Union sent in tanks and crushed the protests, killing 2,500 Hungarians.
During the Cold War, NATO deployed hundreds of U.S. missiles close to the borders of several Warsaw Pact countries, including the Soviet Union. At the same time, NATO leveraged the presence of these warheads to negotiate a dismantling of all short- and intermediate-range missiles with the Soviet Union. Ultimately, the strategy (known as the dual-track decision) was successful: the two sides reached an arms control agreement in 1987. The NATO-sponsored military buildup was risky and controversial among the European public, as it would have made any conflict that did erupt much deadlier.
In 1991, prompted by domestic political reforms and democratic momentum in the countries within its orbit, the Soviet Union disintegrated into Russia and fourteen other countries. This collapse and the dissolution of the Soviet-led Warsaw Pact marked a swift end to the Cold War. NATO had successfully achieved its mission of deterring armed conflict on the continent, keeping the Cold War mostly cold. It is rare for an alliance born in one strategic context to endure once that strategic context fundamentally changes. With the fall of the Soviet Union and the disappearance of its common foe, would NATO dissolve or persevere and take on new missions?
At the end of the Cold War, NATO faced the question of whether it should continue to exist. Rather than retire one of history’s most successful military partnerships, NATO officials decided to reimagine the group’s mission. But many questions remained: Would the alliance stay the same size or admit new members? Would it remain a purely defensive alliance or expand its mission? Over the next decade, NATO expanded its membership and confronted new forms of instability outside its members’ borders.
The first step in reimagining a post–Cold War NATO was an initiative called Partnership for Peace. Its main objectives were forming relationships with former Soviet countries and demonstrating that NATO did not intend to disappear after the Cold War. Although participation in the partnership did not guarantee former Warsaw Pact countries entry into NATO, it was widely seen as a first step toward obtaining membership. Thirteen east European countries—including Russia—worked with NATO allies and cooperated on multilateral activities like humanitarian assistance, peacekeeping, and crisis management. Critics of NATO’s activities in the immediate post-Cold War period believe the alliance should have done more to integrate Russia and provide the country with greater economic assistance, arguing this could have prevented Russian alienation.
In 1995, Bosnian Serbs violated a UN Security Council resolution during the Bosnian War. In response, NATO launched an air campaign against the Bosnian army. In 1999, violence resurged in the region as Serbian authorities persecuted a group of ethnic Albanians who were calling for an independent country. NATO concluded that attacking Serbia was justified in order to prevent mass atrocities. Led by the United States, the alliance launched another extensive bombing campaign. This marked the start of NATO’s transformation from a purely defensive alliance into a large, coordinated, and powerful military force operating beyond its members’ borders.
NATO continued its expansion in 1999, adding the Czech Republic, Hungary, and Poland. The alliance grew again in 2004, extending membership to seven east European countries, many of which were former Soviet republics. The United States largely drove this period of growth, believing that expanding NATO would deter future Russian aggression and provide members with the security they needed to allow for the transition to democracy. However, the expansion was not without consequences. Russia perceived NATO’s creep into Eastern Europe as a direct threat to its security and influence across the region and was alienated by the expansion. And as NATO grew, member countries worried that the additional obligations would stretch the alliance thin.
The September 11, 2001 attacks had a major effect on NATO. Not only did they result in the first—and only—time the alliance invoked Article 5, they also demonstrated the increasing interconnectedness of the world. NATO used this reality as justification to fight threats at their source, even if those were far away from NATO homelands. In the opening years of the new century, NATO adopted a more expansive definition of what it meant to protect its members’ national interests and transformed from a traditional defensive alliance into a collective of like-minded countries willing to pursue broader objectives.
On September 11, 2001, militants from the terrorist group al-Qaeda hijacked four planes and used them as weapons to kill 2,977 people in the United States. The 9/11 attacks prompted the first—and only—invocation of Article 5, under which NATO considered the attacks to be against all of its members. In response, NATO spearheaded an anti-Taliban coalition in Afghanistan, which constituted its first mission outside the North Atlantic. Since then, the mission continuously evolved, until two decades later when NATO forces pulled out in 2021.
NATO’s mission quickly expanded from simply grounding Qaddafi’s air force into a broader political mission to remove Qaddafi from power. In the following months, NATO-backed rebels took over most of Libya and executed Qaddafi. Although NATO leaders hoped Qaddafi’s ouster would mark the end of Libya’s violence, the country soon spiraled into even greater chaos. The unintended consequences of the Libya campaign raised questions about the effectiveness of NATO’s expanding mission. Though NATO was not able to restore democracy in Libya, the goal was to protect Libyan citizens and prevent a massacre. The NATO intervention was able to do just that.
In 2014, Russia annexed the Ukrainian territory of Crimea and began arming separatists fueling conflict in the southeastern part of the Ukraine. This aggression acted as a reminder to NATO countries that Russia still threatened democracy in Europe. NATO responded by suspending all military and civilian cooperation with Russia and by promising to send more troops to its members in Eastern Europe. The need for an alliance to defend Europe's democracies is particularly acute in the Baltic countries of Estonia, Latvia, and Lithuania. They were seen as especially vulnerable to Russian military force as Russia went on to increase its aggression against Ukraine.
Russia fully invaded Ukraine in 2022, kicking off a destructive conflict that has led to the deaths of thousands of people. Wary of Russian aggression, more European leaders turned to NATO for protection, renewing the alliance's strength. Member states also worked together to deliver billions of dollars' worth of military and financial assistance to Ukraine and helped train Ukrainian troops. Ukraine applied to join the alliance, but NATO did not immediately accept, as that would put NATO in direct conflict with nuclear-armed Russia. NATO Secretary General Jens Stoltenberg did announce that all member countries have agreed to allow Ukraine to join the alliance once conflict ends, when it is safer to do so.
NATO gained its thirty-first member in April of 2023 when Finland joined. It and Sweden had applied to become members of the alliance within just months after the Russian invasion of Ukraine. This was an unprecedented move for the two Scandinavian countries. For the entirety of the Cold War, Finland and Sweden had aimed to preserve neutrality and non-alignment, but the Ukraine War caused a major shift in their political calculus. Although Finland successfully joined NATO, Sweden's application was blocked by two other members, Hungary and Turkey in light of lingering political tensions.
