North Atlantic Treaty Organisation (NATO)

This is a defensive military alliance between the USA, and 27 European countries including the UK. It was formed in 1949 ; under the North Atlantic Treaty which established it, an attack against one or more members is considered to be an attack against all. This was invoked in response to the attacks on the USA on 11 September 2001.  The official website of NATO gives more information.

You might also be interested in our other pages on international organisations:

How the international world works