Military or defense alliance formed in 1949 by 12 countries in Western Europe and North America. Original purpose was to protect its members from a possible attack from the Soviet Union. NATO is an alliance of nations with shared democratic values and has been the most important U.S. alliance for almost 60 years. The NATO Treaty includes Article 5 which states that an armed attack against one or more members in Europe or North America is considered an attack against all members. NATO has never had to use its military forces with the exception of the September 11, 2001 attack. The alliance has expanded over the years and includes various countries.