The Soviet nuclear threat posed by a growing bomber fleet in the early 1950s saw the USAF and RCAF partner to build the Pinetree Line network of early warning radar stations across Canada at roughly the 50° north parallel of latitude with additional stations along the east and west coasts. This was expanded in the mid-1950s with the building of the Mid-Canada Line at roughly the 55° north parallel, and finally in the late-1950s and into the early 1960s the DEW Line was built across the Arctic regions of North America. The nature of the Soviet bomber threat and of other hostile incursions into North American airspace saw an RCAF and USAF partnership in the creation of the North American Air (Aerospace, after 1981) Defense Command (NORAD) which was formed on 1 August 1957.