Radar was developed independently in many countries in the 1930s. But, in 1935, Robert Watson-Watt (1892–1973), a Scottish physicist, was the leader of a group that created the first radar defense system for the British military. Although a large number of nations, from the United States and Canada, to Britain, France, and Germany, to the Soviet Union and Japan, worked to develop radar systems during the 1930s, the British system of ground-based radar stations were the first to use radar effectively in warfare. By the early 1940s radar systems were miniaturized enough to be installed in aircraft so that they could engage other aircraft in fights at night.

Ironically, Watson-Watt became a deserving victim of his own technology nineteen years later. According to Canadian police, Watson-Watt had been speeding on a stretch of Canadian road, and was detected by a police radar gun. Watson-Watt willingly paid the fine and drove away.