When the Soviet Union detonated its first atomic weapon in 1949, U.S. defense officials became concerned about a possible Soviet bomber strike against the United States. That same year, the Soviets began production of the Tupolev Tu-4, a four-engine strategic bomber that was a direct copy of the B-29 Superfortress. In response, the United States and its allies accelerated the development of fighter-interceptor aircraft and the deployment of surface-to-air missiles (some with nuclear warheads).
The United States also established a massive radar warning system. That system—eventually dubbed the Distant Early Warning (DEW) Line—stretched across the Northern Hemisphere, with radars in the continental United States, Alaska, Canada, Iceland, Greenland, and Britain to protect against a raid over the North Pole.