 Complete the following phrase:  The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force Luftwaffe used a number of increasingly accurate systems of radio navigation for night bombing in the United Kingdom. British scientific intelligence at the Air Ministry fought back with a variety of their own increasingly effective means, involving jamming and deception signals. The period ended when the Wehrmacht moved their forces to the East in May 1941, in preparation for the 