To defend North America against a possible enemy attack, Canada and the United States began to work very closely together in the 1950s. The North American Aerospace Defense Command (NORAD) created a joint air-defense system. In northern Canada, the Distant Early Warning Line (Dew Line) was established to give warning of Soviet bombers heading over the north pole. Similar early warning radar systems were also developed in the middle of Canada, known as the Mid-Canada Line; and across the 50th parallel north, known as the Pinetree Line.