On May 23, 1967, at the height of the Cold War, the unthinkable nearly happened. At the time, the U.S. relied on radar installations in Alaska, Greenland, and northern UK to provide warning of a first strike by the Soviet Union. Their purpose was to give enough notice for the US to launch its own long-range bombers (or in the worst case, missiles) for a retaliatory strike before they were caught on the ground and obliterated. It was part of a defense strategy called mutually assured destruction, designed to ensure that any attempt at a first strike would be suicidal.