NATO
On April 4, 1949, the United States Canada and 10 Western European countries had signed the Washington Treaty that led to creation of the North Atlantic Treaty Organization. Its original members were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. Article 5 of Washington Treaty is the very heart of it, which mentions that “the members agree that an armed attack against one or more of them in Europe or North America would be considered an attack against all of them”.This idea of collective defense is the basis of NATO.