What Is NATO?

Introduction

Founded in 1949 as a bulwark against Soviet aggression, the North Atlantic Treaty Organization (NATO) remains, seventy-six years later, the pillar of U.S.-Europe military cooperation. An expanding bloc of NATO allies has taken on a broad range of missions since the close of the Cold War, many well beyond the Euro-Atlantic region, in countries such as Afghanistan and Libya.

Russia’s unprovoked invasion of Ukraine, a nonmember, in early 2022 shook Europe’s security architecture and prompted a major reevaluation of NATO members’ foreign policies and defense commitments. The threat from Russia has generated the country’s greatest tensions with the alliance in the post–Cold War era. It is driving up defense spending and has pushed some longtime NATO partners, namely Finland and Sweden, to seek and gain full membership. As the alliance gears up for its 2025 summit in The Hague, members will seek to improve their collective security while ally Ukraine continues its push for accession.

A Post–Cold War Pivot

After the demise of the Soviet Union in 1991, Western leaders intensely debated the direction of the transatlantic alliance. Some in the Bill Clinton administration initially opposed expanding NATO, wary it would upset relations with President Boris Yeltsin’s fragile government in Russia and complicate other U.S. foreign policy objectives, such as nuclear arms control. Others favored expansion as a way to extend NATO’s security umbrella to the east and consolidate democratic gains in the former Soviet bloc.

As a first step, Clinton developed the Partnership for Peace—a nonmembership framework aimed at increasing cooperation between NATO and former Warsaw Pact members, as well as non-European countries—in a bid to allay Russia’s concerns about NATO expansion. However, after the initiative’s launch in 1994, Clinton pivoted to advocating [PDF] for membership expansion. The shift prompted Yeltsin to warn that Europe risked “encumbering itself with a cold peace.”

Beyond Collective Defense

Many U.S. officials felt that a post–Cold War vision for NATO needed to look beyond its core defense commitments—Article V of the North Atlantic Treaty states that “an armed attack against one or more [member states] in Europe or North America shall be considered an attack against them all”—and focus on confronting challenges outside its membership. “The common denominator of all the new security problems in Europe is that they all lie beyond NATO’s current borders,” U.S. Senator Richard Lugar said in a 1993 speech.

The breakup of Yugoslavia in the early 1990s and the onset of ethnic conflict tested the alliance on this point almost immediately. What began as a mission to impose a UN-sanctioned no-fly zone over Bosnia and Herzegovina evolved into a bombing campaign on Bosnian Serb forces that many military analysts say was essential to ending the conflict. In April 1994, NATO conducted its first combat operations in its forty-year history, shooting down four Bosnian Serb aircraft.

NATO invoked Article V for the first and only time following the 9/11 attacks by the Afghanistan-based al-Qaeda terrorist network. Those events ushered in a mission in Afghanistan that—at its height—saw NATO contribute more than 130,000 troops from fifty alliance and partner countries. Analysts say that the mission in Afghanistan marked a turning point for the alliance by signaling that NATO was adapting to the post–Cold War security environment. The United States and NATO allies withdrew their remaining forces—about ten thousand troops—from Afghanistan in 2021, bringing the twenty-year military operation to a close.

NATO’s Structure

Headquartered in Brussels, NATO is a consensus-based alliance in which decisions must be unanimous. However, individual states or subgroups of allies can initiate action outside NATO’s auspices. For instance, the United States, France, and the United Kingdom began policing a UN-sanctioned no-fly zone in Libya in early 2011 before transferring command of the operation to NATO. Member states are not required to participate in every NATO operation; Germany and Poland declined to contribute directly to the campaign in Libya.

NATO’s military structure comprises two strategic commands: the Supreme Headquarters Allied Powers Europe, located near Mons, Belgium, and the Allied Command Transformation, located in Norfolk, Virginia. The supreme allied commander Europe oversees all NATO military operations and is always a U.S. flag or general officer; U.S. Army General Christopher G. Cavoli currently holds this position. Although the alliance has an integrated command, most forces remain under their respective national authorities until NATO operations commence.

NATO’s secretary-general, former Dutch Prime Minister Mark Rutte, is the alliance’s civilian leader. Rutte took office in October 2024, succeeding Norwegian politician Jens Stoltenberg, who had led the alliance for a decade. Stoltenberg championed NATO’s expansion and support for Ukraine in the wake of Russia’s full-scale invasion. The secretary-general is also charged with chairing NATO’s principal political body, the North Atlantic Council, which is composed of high-level delegates from each member state.

Sharing the Burden

Member states’ primary financial contribution is the cost of deploying their respective armed forces for NATO-led operations. These expenses are not part of the formal NATO budget, which funds alliance infrastructure, including civilian and military headquarters, and stands at about $5.3 billion in 2025. NATO members were estimated to have collectively spent about $1.3 trillion [PDF] on defense in 2024. The United States accounted for roughly 64 percent of this, up from about half during the Cold War.

For years, many NATO members failed to meet the defense spending target of 2 percent of gross domestic product (GDP) that was agreed to in 2014, but Russia’s 2022 invasion of Ukraine sparked a change. In 2024, twenty-three of the alliance’s thirty-two members met or exceeded the 2 percent target, up from just six in 2021. 

While U.S. officials have regularly criticized European members for failing to meet their budget commitments to NATO, U.S. President Donald Trump has taken a more assertive approach, suggesting his administration would reexamine U.S. treaty obligations if the status quo persisted. The number of members meeting their spending pledges increased slightly during Trump’s first term, although some subsequently slipped below the 2 percent threshold. In his second term, the Trump administration is calling on NATO members to increase their defense spending contributions to 5 percent of their GDP. Of that 5 percent, 3.5 would be spent on core defense spending, while an additional 1.5 percent would be spent on defense-related expenditures.

Russia’s 2022 military assault on Ukraine, the largest land war in Europe since World War II, shocked many European defense planners and has led many alliance members—most notably Germany—to significantly increase their military spending. In the weeks after Russia’s invasion, then-German Chancellor Olaf Scholz pledged to boost weapons investments by one hundred billion euros and exceed NATO’s 2 percent threshold by 2024. Germany confirmed in January 2025 that it had successfully done so.

Tensions With Russia

Moscow has long viewed NATO’s post–Cold War expansion into Central and Eastern Europe with great concern. Many current and former Russian leaders believe the alliance’s inroads into the former Soviet sphere are a betrayal of alleged U.S. guarantees to not expand eastward after Germany’s reunification in 1990, although U.S. officials involved in these discussions dispute this characterization of history.

Most Western leaders knew the risks of enlargement. “If there is a long-term danger in keeping NATO as it is, there is immediate danger in changing it too rapidly. Swift expansion of NATO eastward could make a neo-imperialist Russia a self-fulfilling prophecy,” wrote Secretary of State Warren Christopher in the Washington Post in January 1994.

Over the years, NATO and Russia took significant steps toward reconciliation, particularly with the signing of the 1997 Founding Act, which established an official forum for bilateral discussions; however, a persistent lack of trust has plagued relations.

NATO’s Bucharest summit in the spring of 2008 deepened suspicions. While the alliance delayed membership action plans for Georgia and Ukraine, it vowed to support their membership down the road, despite Russia’s repeated warnings of political and military consequences. Russia’s invasion of Georgia that summer was a clear signal of Moscow’s intentions to protect what it sees as its sphere of influence, experts say.

Russia’s annexation of Crimea in 2014 and its continued destabilization of eastern Ukraine further spoiled relations with NATO. Weeks after the annexation, NATO suspended all civilian and military cooperation with Moscow.

President Trump entered office in 2017 aiming to ease tensions with Russian President Vladimir Putin. However some members of his administration, as well as many in the U.S. Congress and military, resisted this effort given what they saw as Russia’s ongoing transgressions, most notably its attempts to meddle in foreign elections and develop new nuclear weapons. Late in his presidency, Trump planned to restructure the U.S. military posture in Europe, which would have seen a reduction of its overall footprint there, but this did not materialize. President Joe Biden, on the other hand, sought to bolster the transatlantic alliance during his term from 2021 to 2025.

Russia-NATO tensions came to a head in late 2021 and early 2022 when Putin ordered an extraordinary military buildup on the border with Ukraine and threatened a wider invasion unless the alliance pledged to permanently stop expanding its membership, seek Russian consent for certain NATO military deployments, and remove U.S. nuclear weapons from Europe, among other guarantees. Alliance leaders dismissed these requests while seeking other diplomatic avenues, and Russia launched its invasion in February 2022.

A Renewed Alliance

Years of Russian aggression in Ukraine have pushed the alliance to reinforce defenses on NATO’s eastern flank. Since its 2014 summit in Wales, NATO has ramped up military exercises and opened new command centers in Bulgaria, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, and Slovakia. The outposts are intended to support a new rapid reaction force of about twenty thousand, though NATO military planners say that a multinational force of about forty thousand could be marshaled in a major crisis.

In 2017, NATO began rotating four multinational battle groups through the Baltic states and Poland and bolstered defenses in the Black Sea region, creating a new multinational force of several thousand in Romania. In addition, the alliance increased its air patrols over its eastern borders and routinely scrambles jets to intercept Russian warplanes violating allied airspace. Meanwhile, the U.S. Army added another rotational armored brigade to the two it had in the region.

NATO members have increasingly collaborated with Ukraine, although as a nonmember, Ukraine remains outside of NATO’s defense perimeter. In the years leading up to Russia’s invasion, Ukraine held annual military exercises with the alliance and became one of just six enhanced opportunity partners, a special status given to the bloc’s closest nonmember allies. Since the start of the war, Kyiv has repeatedly affirmed its goal to eventually gain full NATO membership, though some experts say that is unlikely.

The Look Ahead

Since Russia’s 2022 invasion, many NATO member countries—including the United States—have provided Ukraine with an unprecedented amount of military support, including sophisticated weaponry such as tanks, heavy artillery, armed drones, and antiaircraft systems. This lethal aid is not committed under alliance auspices, and NATO leaders have been keen to avoid taking actions, such as implementing a no-fly zone, that could draw it into direct conflict with Russia or otherwise escalate hostilities. Still, Russia has warned that in providing this assistance, NATO allies are risking the outbreak of a nuclear war. 

Since Trump’s return to office in January 2025, the United States has softened its support for Ukraine, temporarily pausing some military aid to the country and often exerting more pressure on Kyiv than Moscow to make concessions to secure a ceasefire. At the Munich Security Conference in February, U.S. Defense Secretary Pete Hegseth dismissed the prospect of Ukraine’s NATO accession and ruled out the possibility of a U.S.-led peacekeeping mission in Ukraine. The fracturing of U.S. support for Ukraine also coincides with some divides within NATO about the alliance’s own support for the country. Despite NATO leaders’ pledges to support a pathway for Ukraine’s entrance into the alliance, members like Hungary and Slovakia have both expressed opposition to the idea. These developments also coincide with Trump’s mutable support for the tenants of NATO’s collective defense; in March 2025, he suggested the United States would not defend member states who were not contributing enough towards defense spending.

Yet Russia’s provocations also prompted another major NATO expansion. Finland and Sweden, two countries with a history of avoiding formal military alignment, applied to join the alliance in 2022. Finland acceded in April 2023, expanding NATO’s Nordic footprint and doubling the length of its border with Russia. Sweden’s bid was delayed by political disputes with Turkey and Hungary, but it became a member in March 2024 after the two holdouts dropped their objections.

NATO will hold its 2025 summit in The Hague, Netherlands, from June 24–26. NATO allies are expected to address major areas of concern during the meeting, including defense spending, support for Ukraine, and the alliance’s defense posture. NATO chief Rutte said in early June that he expects the alliance to approve a four hundred percent increase to NATO’s air and missile defenses during the summit to ward against potential future attacks. In a sign of NATO’s continued close relationship with Ukraine, Ukrainian President Volodymyr Zelenskyy has been invited to the gathering and is expected to attend.

Recommended Resources

In this policy brief, CFR experts Liana Fix and Rebecca Lissner argue that European leaders should focus on increasing defense spending and planning for a European-led future for NATO.

For CFR’s Securing Ukraine’s Future initiative, Eugene Rumer explores how Ukraine could develop a security policy in the absence of NATO accession. 

Congress’ current proposal for additional sanctions on Russia could tank the global economy—but there are ways to strengthen the bill, CFR’s Stephen Sestanovich writes.

In this expert brief, CFR expert Michael C. Horowitz explains how Ukraine’s recent drone attacks on Russia could shape the country’s expectations for the war going forward and illustrate the future of global warfare. 

For Foreign Affairs, the European Council on Foreign Relations’ Jeremy Shapiro makes the case that NATO’s supreme allied commander for Europe should be European—not American—to show the continent’s willingness to assume responsibility for its own defense.