An attack on one is an attack on all: Article 5 of the North Atlantic Treaty, invoked just once

Background

New security pacts around the world are often described as carrying a mutual defence clause, and the model for such clauses is Article 5 of the North Atlantic Treaty. The Article says that an armed attack against one or more members in Europe or North America shall be treated as an attack against all of them, and that each member must then come to the help of the member attacked. The treaty that carries this Article was signed on 4 April 1949 in Washington, D.C. In the whole history of the alliance, Article 5 has been invoked only once, after the attacks of 11 September 2001 on the United States. The alliance's decision-making body, the North Atlantic Council, decided the day after the attacks that they would be covered if directed from abroad, and on 2 October 2001 it determined that they were. The clause is what makes the alliance a collective defence arrangement rather than a loose grouping.