 In the post @-@ Second World War period , the Soviet Union began developing a capable fleet of long @-@ range bombers with the ability to deliver nuclear weapons across North America and Europe . The main threat was principally from high @-@ speed , high @-@ altitude bombing runs launched from the Soviet Union travelling over the Arctic against military bases and built @-@ up industrial centres in Canada and the United States . To counter this threat , Western countries strenuously undertook the development of interceptors that could engage and destroy these bombers before they reached their targets . 
