In WWII the British were always trying to find a way to get past German radar and fighter defenses so their bombers would be safer. In 1942 they found a very simple way that worked that they called ‘Window’. It was first proposed in 1937 and it involved strips of tin-foil of a certain size that would be released from the bomber. It created an effective defense by not allowing the Germans to figure out in a timely manner which was the bomber and which was a strip of metal. The problem that the British realized though was that the solution was so simple that the Germans could copy it and use it against them since they also did not know how to see through this defense. After the war the Allies found out that the Germans had also thought of this idea but didn’t use it for fear the British would use it against them. By the time the British decided to use it in 1943 the German bomber force was so small that they were not able to use it with the same effectiveness as the British.

(Book – The Second World War Volume 4 – Winston s. Churchill)