In 1904, Christian Hulsmeyer demo-ed to the German Navy a ‘Hertzian wave projection and receiving device to detect the presence of a metallic body’. Hulsmeyer failed to excite any interest and turned his mind to other matters.
In January 1935, the UK Air Ministry formed the Committee for the Scientific Study of Air Defence and approached Robert Watson-Watt of the National Physical Laboratory who started looking at using electro-magnetic waves to detect aircraft.
Before the end of 1935, Watson-Watt had demo-ed continuous-wave radar and pulse radar.
The UK immediately started building a chain of radar stations of which the first became operational in 1937. By 1939 the UK had 25 radar stations and, by 1945, 50.
The radar stations and the RAF stopped Hermann Goering from establishing the air supremacy necessary to allow the German Army to cross the Channel in an invasion of Britain called Seal Lion planned by Adolf Hitler.