Finland becomes 31st country to join Nato. Finland has officially joined Nato in a historic moment for the country About NATO: North Atlantic Treaty Organization (NATO) is a military alliance established by the North Atlantic Treaty (also called the Washington Treaty) of April, 1949, by the United States, Canada, and several Western European nations to provide collective security against the Soviet Union. There are currently 31 member states. Headquarters: Brussels, Belgium. Headquarters of Allied Command Operations: Mons, Belgium.