The essential purpose of the North Atlantic Alliance is to safeguard the freedom and security of all its members in Europe and North America in accordance with the principles of the United Nations Charter. To achieve this, the Alliance uses both its political influence and its military capacity, depending on the nature of the security challenges facing Alliance member states. As the strategic environment has changed, so too has the way in which the Alliance responds to security challenges. It continues to preserve stability throughout the Euro-Atlantic area and is evolving to meet new threats such as terrorism and other security challenges beyond its traditional area of responsibility.
The North Atlantic Treaty Organisation (NATO) is one of the key structures through which Alliance members implement their security goals. It is an intergovernmental organisation in which member countries retain their full sovereignty and independence, and serves as a forum in which they consult together and take decisions on matters affecting their security. NATO's structures facilitate continuous consultation, coordination and cooperation between members on political, military, economic and other aspects of security, as well as cooperation in non-military fields such as science, information, the environment and disaster relief.
After five rounds of enlargement, NATO's 12 founding members - Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States - have been joined by Greece and Turkey (1952), Germany (1955), Spain (1982), the Czech Republic, Hungary and Poland (1999), Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia (2004), and in the most recent round of enlargement Albania and Croatia (2009).
(From NATO-publication, NATO Transformed (PDF ~1500Kb)