When did the project Operation Big was related to begin?
Operation Big: Operation Big was an operation of the Alsos Mission, the Allied seizure of facilities, materiel, and personnel related to the German nuclear weapon project during World War II. German nuclear weapon project: The German nuclear weapon project (German: "Uranprojekt" ; informally known as the "Uranverein"; English: Uranium Society or Uranium Club ) was a scientific effort led by Germany to develop and produce nuclear weapons during World War II. German nuclear weapon project:  The first effort started in April 1939, just months after the discovery of nuclear fission in December 1938, but ended only months later due to the German invasion of Poland, after many notable physicists were drafted into the "Wehrmacht".
April 1939