He served as leader of the Progressive Conservative Party (1956–67) and Prime Minister of Canada (1957–63). He introduced some important measures of social reform and sought to encourage economic development, but as Canada experienced increasing economic difficulties in the early 1960s he was forced to devalue the Canadian dollar. In foreign affairs Diefenbaker wished to reduce Canada’s dependence on the USA, but his party lost the election of 1963 when he took issue with the USA over the arming with atomic warheads of missiles supplied to Canada.