On April 4, 1949, less than four years after the end of World War II, 12 nations from Europe and North America signed the Washington Treaty and NATO was born. This group of one dozen nations — Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the UK and the US - understood the great strength of collective defense and the vast potential that defense cooperation could bring for peace and security of the Euro-Atlantic area.

In addition to being the founding document for NATO’s concept of collective defense, the Washington Treaty also includes specific articles that provide a framework for how it is realized by allied states. At the heart of the North Atlantic Treaty is the solidarity expressed in Article 5, which states that an attack against one ally is considered as an attack against all allies.