NATO’s Article 5 is central to the alliance. The signatory members agree that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” The article has only been invoked one time: in 2001, after the September 11 attacks organized by Osama bin Laden destroyed the World Trade Center in New York City and part of the Pentagon outside Washington, D.C., killing some 3,000 people.