Here is all the info you need on it: http://en.wikipedia.org/wiki/NATO

The North Atlantic Treaty Organization (NATO) was formed in Washington on 4 April 1949. Made up of nations from North America and Europe, 12 independent countries originally signed the North Atlantic Treaty. The treaty's purpose was to commit each signatory country to the peace and security of all other members. A total of 28 countries are now members. Since the Cold War ended, the structure of the treaty has been redesigned. The emphasis is now maintaining peace through the treaty's political and military guidelines. Other countries who are not members of NATO also cooperate, although sometimes unsuccessfully, with treaty guidelines. Apart from peacekeeping between countries and deterring war, NATO also acts in the area of crisis management. NATO has guidelines set out in order to prevent conflict between countries and to help resolve any crisis.

北大西洋・公约・组织 Started by the countries of Western Europe and North America. If one of them is attacked, the others have to help out. The Russians and Eastern Europeans had their Warsaw Pact. That is now defunct.

北约——北大西洋公约组织