They were converted from mothballed River-class frigates of British design that had been placed in reserve following the end of the Second World War.

It was believed at the time that there was a strategic threat to the shipping lanes supplying the European continent by the Soviet Union.

This belief originated due to Soviet submarines becoming increasingly difficult to detect and identify, as a result of their updated technology.

Originally that meant updating only the existing fleet, however this policy was expanded when 21 decommissioned River-class frigates were converted to ocean escorts and recommissioned into the Royal Canadian Navy.

[1] The concept of anti-submarine warfare performed from a helicopter operating from the decks of escorts had been first proposed during the Second World War.

[1] HMCS Buckingham had a helicopter flight deck attached to her stern and performed sea trials from October to December 1956.