Which countries formed a treaty called Five Eyes for joint cooperation in signals intelligence?

The Five Eyes (FVEY) is an intelligence alliance comprising Australia, Canada, New Zealand, the United Kingdom and the Untied States. These countries are parties to a multilateral UKUSA Agreement (The United Kingdom-United States of America), a treaty for joint cooperation in signals intelligence.

Signals intelligence (SIGINT) is intelligence-gathering by interception of signals, whether it is communications between people or from electronic signals not directly used in communications. Often the information is sensitive and encrypted. Encryption means it is coded by converting plaintext into an alternative form known as ciphertext.

The origin of the FVEY can be traced to the Atlantic Charter issued by the Allies in 1941 to lay out their goals for a post-war world. Shortly after the end of WWII (1945), the Cold War started and the ECHELON (originally a secret government code name) was started. It was a surveillance program initiated to monitor the communications of the former Soviet Union and the Eastern Bloc countries (a group of communist states in Central and Eastern Europe, East Asia and Southeast Asia).

The FVEY further expanded their surveillance capabilities during the course of the ‘war on terror’, with an emphasis placed on monitoring the World Wide Web.

In spite of continued controversy over its methods, the Five Eyes relationship remains one of the most comprehensive known espionage alliances in history.