NATO's Article 5 has been invoked once—after 9/11

How NATO responded to 9/11

The collective defense clause of the NATO constitution has only been invoked one time: after the 9/11 terrorist attacks against the United States. This video explores the immediate actions NATO took to respond to the attacks to support the United States. These firsthand clips from 2001 give a sense of the magnitude of the moment and the difficult decisions facing NATO leaders at the time.

The treaty's Article 5 requires mutual defense, but not necessarily military action

Article 5 of the North Atlantic Treaty established NATO’s principle of collective defense. This resource examines the historical context, legal interpretations, and practical applications of this commitment, including its sole invocation after the Sept. 11 attacks.

Members decide to what extent they respond to an attack on an ally

The NATO constitution is intentionally vague about what exactly is required from its members. Explore the historical debates and learn more about common misconceptions about NATO’s common defense clause.

The alliance operates under a "flexible response" doctrine—that is, proportionality

NATO countries' main deterrent against aggression is their ability to respond with nuclear weapons. This video from "The Infographics Show" outlines the purpose of this nuclear arsenal, NATO’s strategy to prevent attacks, and its plans in the event of escalation toward nuclear war.

How NATO works

NATO now includes 32 member nations—but how does the alliance actually function? This explainer breaks down its structure, decision-making process, and funding model. From its founding in 1949 to its collective response after 9/11, here’s how NATO operates on a global scale.

NATO was formed in 1949 to deter Soviet expansion and integrate Europe

Europe was in shambles after World War II, which saw 36 million Europeans killed, and the Soviets were looking to increase their influence. Beyond deterring the Soviets, the alliance sought to restore Europe and prevent nationalist militarism akin to the German Nazis and Italian Fascists.

Watch the historic treaty signing that established NATO in 1949

NATO was created with the signing of the North Atlantic Treaty on April 4, 1949, in Washington, DC. The clip embedded in the webpage includes a newsreel with an excerpt from US President Harry Truman’s speech celebrating the historic day.

The 2001 Authorization for Use of Military Force has been used by four administrations to justify military operations in 22 countries

Passed three days after the September 11 attacks to authorize the use of force against al-Qaeda and the Taliban, the 2001 AUMF has since been stretched far beyond its original scope and has been used by four consecutive administrations to justify military operations.

NATO condemned Russia's 2022 invasion of Ukraine

While Ukraine is not a member, NATO responded forcefully to the Russian invasion of the country in 2022. This page from the NATO website offers answers to frequently asked questions about NATO’s response to the invasion, specifically on how NATO is aiding Ukraine.

NATO leads cyber war games in Estonia

Estonia is host to one of the world’s largest annual interactive cybersecurity drills. Russia is always considered the main threat, but this year, the war in Ukraine has only added to the stakes.

Chernobyl pushed the world to create rapid-alert systems for nuclear accidents

In the aftermath of the disaster, nations adopted new agreements requiring immediate international notification of nuclear accidents. They coordinated emergency assistance—efforts led by the International Atomic Energy Agency to prevent future disasters from being concealed when radiation emergencies occur.

Congress has formally declared war 11 times across five conflicts, the last in 1942

The five declared wars were the War of 1812, the Mexican-American War, the Spanish-American War, World War I, and World War II. Every major American military conflict since Korea, Vietnam, Iraq, and Afghanistan has been fought without a formal declaration.

The attacks prompted a quick jump in American public support for war

In a Gallup poll taken days after the attack, an extraordinary 97% of Americans approved Congress declaring war on Japan—reflecting overwhelming national unity in the wake of the strike.

General Jennie Carignan was the first woman to lead a NATO mission

"NATO Through Time" is a podcast exploring NATO’s history and its impact on today’s world. This episode features General Jennie Carignan, Canada’s top military leader, as she shares insights from her NATO missions in Bosnia, Afghanistan, and Iraq.

Franklin D. Roosevelt's 1941 speech framed World War II as a global fight for universal rights

Eleven months before the attack on Pearl Harbor, President Roosevelt outlined four fundamental freedoms that, in his words, people "everywhere in the world" ought to enjoy—speech, worship, freedom from want, and freedom from fear.

After the Sept. 11 terrorist attacks, The Onion stuck with satire

Although other comedians believed anything lighthearted in the wake of the attacks was inappropriate, The Onion, a satirical newspaper, decided to do what it always did. That issue’s top story was a headline that would go down in comedy history.

Five key causes of World War I

What caused World War I? The ready answer refers to the assassination of Archduke Franz Ferdinand. But there was more to it than that. This article, accompanied by a short explanatory video, outlines how mutual defense alliances, imperialism, militarism, and nationalism also had a heavy hand in the start of what some considered a "preventable war."

The president can federalize state National Guard units during national emergencies

While the governor usually commands their state's National Guard, the president can assume control—federalizing the guard—under laws like the Insurrection Act or Title 10. Once federalized, troops operate under the US military chain of command.

The 5 permanent Security Council members can veto resolutions

The UN Security Council veto is a power held by its five permanent members—China, France, Russia, the UK, and the US—allowing any one of them to block resolutions, regardless of international consensus. It has been used over 300 times since the UN's founding.

The Berlin Airlift was the largest in history and a major Cold War victory for the US

In 1948, the USSR blockaded West Berlin to pressure America and its allies. In response, the US and its partners launched the largest airlift in history—delivering supplies daily for nearly a year until Stalin backed down.

Infographics explaining global terrorism trends

Our World in Data’s terrorism page details global trends in terrorism. Through interactive infographics, the site shows how the number of terrorist acts varies around the world and how it has changed over time. Sections investigate common definitions of terrorism, trends in global terrorism deaths, and the global distribution of terrorism. Additional segments highlight airline hijackings, public opinion, and media coverage of terrorism. Explore this website for an array of infographics on terrorism.

Spikes in pizza orders near US government buildings are thought to be signs of international crises

Theorists have long believed that sudden increases in pizza deliveries near the Pentagon signal a major event is about to occur. Historical examples include the CIA ordering 21 pizzas the day before the beginning of the Gulf War, and another surge of deliveries before former President Bill Clinton's impeachment hearings. Citizens have since begun monitoring food orders in the area. See a tracker of Pentagon pizza orders here.

The CISA’s strategy for secure 5G

CISA, through the National Risk Management Center (NRMC), is leading risk mitigation efforts by working with government and industry partners to ensure the security and resiliency of 5G technology and infrastructure.

The G20 emerged from the 1997 Asian financial crisis as a forum for managing the global economy

Established in 1999 and elevated to a leaders' summit during the 2008 financial crisis, the G20 now unites the world's largest economies to address issues that the Bretton Woods institutions were not designed to manage.

Kurt Vonnegut's 'Cat's Cradle' was inspired by the bombing of Hiroshima

The writer fought in World War II and witnessed the firebombing of Dresden, which he recounted in 1969's satirical "Slaughterhouse Five." Six years earlier, though, he wrote about the bombing of Hiroshima in "Cat's Cradle," channeling some of the horror he witnessed in Dresden to write about the dropping of the atomic bomb. (Some users may experience a paywall.)

The Russia-Ukraine conflict is the largest ground war in Europe since World War II in casualties, forces mobilized, and displacement of civilians.

A 2019 Facebook event called on attendees to "Storm Area 51, They Can't Stop All of Us," resulting in countless memes, the creation of a music festival, and eventually the arrests of those who did try to approach the area.

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

View the ten deadliest tornadoes in US history.

The US first hosted the World Cup in 1994, a tournament that lives in soccer infamy for its poor planning and the murder of Colombian player Andres Escobar, who'd accidentally scored on his own net.

In 1984, Operation Smash Hit intentionally crashed a 239-ton train into a nuclear waste canister at 100 miles per hour to prove the canister's safety, which remained completely intact and lost just 0.29 of its 100 pounds of pressure.

In 2025, four executives from large tech companies were given positions in the US Army Reserves—including Palantir Chief Technology Officer Shyam Sankar, which reinforced Palantir's ties with the US military and its role as a leading defense partner.

US intelligence allegedly pursued more than 600 schemes to assassinate Cuban revolutionary Fidel Castro.

While the first exchange-traded fund (a 'basket' of different assets that can be bought and sold on a stock exchange) in the US only launched in 1993, the global ETF market has grown to more than $10T in the short time since.

Explore an interactive map of battles from the American Revolution, along with detailed summaries of each engagement

See these 16 pieces of art evoking phobias of snakes, flying, spiders, and more. 

As of 2025, the most cited papers in science include a review of graphene, methods to improve image recognition, streamlining electron density calculations, and optimizing protein studies.

The idea for the Federal Reserve was conceived in secret in 1910—with bankers donning duck hunt gear for disguise while traveling to a remote country club on Georgia's Jekyll Island

Iran's Revolutionary Guard Corps is independent of Iran's regular army and is one of the most powerful and influential military-political organizations in the Middle East.

The identity of the Tank Man in the iconic photo from the 1989 Tiananmen Square protests remains unknown and was possibly killed.

The most expensive ad of all time was for Chanel No. 5 perfume and cost roughly $33M to produce in 2004—about $57M in today's dollars.

Astronauts have repaired genetic damage in space using CRISPR—a molecular tool that can make precise changes to DNA and is used in single-celled organisms to attack invasive viruses.

American dance legend Martha Graham, sometimes called 'the Picasso of dance,' refused to perform at the 1936 Olympics in Berlin and conceived the piece 'Chronicle' in response to the experience.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Take a 3D tour of the Hajj, Islam's annual pilgrimage to Mecca, undertaken by over 1.5 million Muslims.

Power lines are dotted with large, brightly colored balls to improve the visibility of these thin wires to helicopters and small planes and help prevent potential crashes.

The first image of a black hole was captured by creating an Earth-sized virtual observatory from a network of eight radio telescopes.

A small portion of The Winstons' 'Amen, Brother,' an obscure '60s gospel record, is the basis for thousands of hip-hop tracks, making it one of the most sampled records ever that's informally known as 'the Amen break.'