When the text founding the North Atlantic Treaty Organization (NATO) was signed in Washington on April 4, 1949, the objective of the 12 founding members (in alphabetical order: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States) was simple: to set up a mutual assistance mechanism to contain any expansionist ambitions of the Soviet bloc in Europe at the start of the Cold War.

Over the past three-quarters of a century, NATO has grown in membership – it now counts 32 allies – but the overall structure has remained unchanged. And the commitment to mutual solidarity of its members remains as strong as ever in the provisions of Article 5 of the treaty. This article states that if a NATO country is the victim of an armed attack, each Alliance member will consider this act of violence as an armed attack against all members and will take such measures as it deems necessary, "including the use of armed force," to come to the aid of the attacked country.

There's nothing automatic about this article. If a country feels attacked, a discussion takes place within the North Atlantic Council and a unanimous political decision must be reached after discussion between all the allies. The fact that this principle never had to be invoked during the Cold War can be seen as proof of its deterrent effect. Without this single, immutable principle, the treaty is meaningless.

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