The North Atlantic Treaty Organization (NATO) is a military alliance that consists of 28 member states from North America and Europe. It was established at the signing of the North Atlantic Treaty on 4 April 1949. Article Five of the treaty states that if an armed attack occurs against one of the member states, it should be considered an attack against all members, and other members shall assist the attacked member, with armed forces if necessary.
Of the 28 members of NATO, two are North American countries and 26 are European countries. The founding members included Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, United Kingdom and the United States. Most recently, Albania and Croatia joined the Alliance on April 1, 2009.