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The article, as the cornerstone of a charter signed in 1949, establishes solidarity among member states and has been invoked only once.

Article 5 is the cornerstone of the North Atlantic Treaty Organization (NATO)  and states that an attack on one member of NATO is an attack on all of its members. But despite its importance, NATO has only invoked Article 5 once in its history—in response to the terrorist attacks of September 11, 2001.  

NATO and Article 5 were established in 1949 in the aftermath of World War II when communist movements supported by the Soviet Union posed a serious threat to democratically elected governments all over a devastated Europe. In 1948, the Communist Party of Czechoslovakia overthrew that nation’s democratic government, while in Germany, Soviet authorities blockaded the Allied-controlled section of Berlin in an attempt to strengthen their position there.

The Berlin Airlift, when U.S. and British planes carried food, fuel and other vital supplies to the isolated citizens of West Berlin, marked an early victory for the West in the Cold War. And with the launch of the Marshall Plan, which provided economic aid to the war-ravaged countries of Europe, the United States had decisively abandoned its earlier policy of isolationism.

But at such a vulnerable time, it seemed clear that Europe required not just economic aid, but also military support, in order to counterbalance the power of the Soviet Union, prevent the revival of nationalist military movements (such as Nazism) and allow for political development along democratic lines.

VIDEO: The Formation of NATO

Tensions between the United States and the Soviet Union resulted in the formation of key alliances that would endure throughout the Cold War.

In April 1949, representatives from 12 nations—the United States, Canada, Great Britain, France, Belgium, the Netherlands, Norway, Denmark, Luxembourg, Iceland, Italy and Portugal—gathered in Washington, D.C. to sign the North Atlantic Treaty.

“Men with courage and vision can still determine their own destiny,” President Harry S. Truman declared at the signing ceremony. “They can choose slavery or freedom—war or peace…If there is anything certain today, if there is anything inevitable in the future, it is the will of the people of the world for freedom and for peace.”

The treaty’s key provision was Article 5, which began: “The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all…” While this commitment to collective defense lay at the heart of NATO, it was left to the judgment of each member state to decide how exactly it would contribute.

On September 12, 2001, the day after the terrorist attacks at the World Trade Center and Pentagon, NATO invoked Article 5 for the first time in its history, committing its members to stand by the United States in its response to the attacks. In a four-paragraph resolution that passed unanimously, the organization reflected its understanding that the threats to global security had changed radically in the 52 years since the alliance was founded.

AUDIO: NATO Offers Aid to United States Following 9/11 Attacks

On October 2, 2001, NATO Secretary-General Lord Robertson holds a press conference to discuss the events of September 11, and pledges support of the 18 NATO allies in the campaign against international terrorism.

”The commitment to collective self-defence embodied in the Washington Treaty was entered into in circumstances very different from those that exist now,” the statement read. “But it remains no less valid and no less essential today, in a world subject to the scourge of international terrorism.”

In addition to participation in the war in Afghanistan, NATO’s response to the 9/11 attacks under Article 5 included Operation Eagle Assist, in which NATO aircraft helped patrol the skies over the United States for seven months between 2001 and 2002, and Operation Active Endeavour, in which NATO naval forces were sent to perform counterterrorism activities in the Eastern Mediterranean. Operation Active Endeavour, which began in October 2001 and later expanded to the entire Mediterranean region, didn’t conclude until 2016.  

Though Article 5 has only been officially invoked once, NATO has taken collective defensive measures in other situations, including deploying missiles on the border of Turkey and Syria in 2012. Russia’s annexation of Crimea in 2014 and the rise of ISIS in recent years led the organization to implement a huge increase in its collective defenses, including tripling the size of the NATO Response Force. In 2014, NATO member states agreed to try and spend 2 percent of their GDPs on defense, although most member states fail to meet this non-binding goal.

President Trump has been highly critical of NATO, calling it “obsolete” and criticizing other NATO members for not spending enough on defense. But he also affirmed U.S. commitment to Article 5 in June 2017, during a news conference with the president of Romania: “I’m committing the United States to Article 5, and certainly we are there to protect, and certainly that’s one of the reasons that I want people to make sure we have a very, very strong force by paying the kind of money necessary to have that force.”

Despite this commitment, Trump appeared to question U.S. responsibility to defend the newest of NATO’s 29 member states, under Article 5 during a Fox News interview in July 2018. In response to a question about whether US forces should respond if Montenegro were attacked, Trump said that the tiny nation’s “very aggressive people” might end up drawing NATO’s members into a war with Russia.

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