The United Kingdom – United States of America Agreement is a multilateral agreement for cooperation in signals intelligence among the United Kingdom, the United States, Canada, Australia, and New Zealand. The alliance of intelligence operations is also known as Five Eyes. It was first signed in March 1946 by the United Kingdom and the United States and later extended to encompass the three Commonwealth realms of Canada, Australia and New Zealand. The UKUSA Agreement was a follow-up of the 1943 BRUSA Agreement, the World War II agreement on cooperation over intelligence matters. This was a secret treaty, allegedly so secret that it was kept secret from the Australian Prime Ministers until 1973.
The agreement established an alliance of five English-speaking countries for the purpose of sharing intelligence, especially signals intelligence. It formalized the intelligence sharing agreement in the Atlantic Charter, signed in 1941, before the entry of the U.S. into the conflict.