Sir Robert Watson-Watt (1892-1973) was born on this date.
Watson-Watt invented radar in 1935, specifically for the purpose of detecting aircraft. His "pioneering work…resulted in the design and installation of a chain of radar stations along the East and South coast of England in time for the outbreak of war in 1939. This system…provided the vital advance information that helped the Royal Air Force to win the Battle of Britain." (From Radar Personalities: Sir Robert Watson-Watt.)
Had the Battle of Britain gone the other way, Hitler probably would have invaded England. A successful invasion would have sundered the U.S.-British alliance and ensured Hitler's victory in Europe.