During the second world war, it was crucial for every engaged party to get to know as much information as possible about the opponent. Without going into detail, an important success for the Allies was to break the code of the German Enigma machines. Achieving this resulted in detailed information about the German forces and probably shortened the war.
It shows both how important it is to have proper protection against opponents trying to gain secret information and how important it is to try to obtain these secrets.
A secret agreement in 1943 between Britain and the United States lead to an alliance to exchange cryptographic systems and knowledge about German and Japanese ciphers. In 1947, this was supplemented by the UKUSA (United Kingdom - USA) Agreement which enclosed besides the USA and Britain Canada, Australia, and New Zealand. This agreement still exists today and is the foundation for the Echelon system I will describe in section 4.4 (, cited in ).
During the Cold War both the Western and Eastern countries engaged mainly in military espionage against the other side. With the existence of more modern technology the systems became more sophisticated. Most of the current systems were designed and implemented during that time.