The Pinetree Line was built to control the air battle between the NORAD interceptor forces and manned Soviet bombers. Beginning with Ground-controlled interception updated from the Second World War, the system has been computerized and automated with at least four new generations of technology being employed. It was clear, even in the early years of the Cold War, that on paper, Canada and the US were to be jointly responsible for the defence of the continent. In execution, Canadian investment in air defence has decreased significantly with the decline of the intercontinental strategic bomber threat. In the 1950s the RCAF contributed fourteen squadrons of CF-100 interceptors and this was reduced to three squadrons of CF-101s by 1970. Some of this is due to improved technology but more is due to the decline of the bomber threat and reductions in Canadian military spending.