Brigadier General Leslie R. Groves was the military head of the Manhattan Project, and J. Robert Oppenheimer was the scientific director. Physicists Enrico Fermi and Arthur Compton first successfully split atoms in a controlled environment and obtained the desired chain reaction.
Columbia University, The University of California Berkeley, and Chicago University were the initial research institutions involved in the Manhattan Project. Economist Alexander Sachs helped convince President Roosevelt of the need for the project, and Vannevar Bush of the Carnegie Foundation was appointed chairman of the National Defense Research Committee in 1940, before the project officially became the Manhattan Project. Research spread from the initial three sites to include Los Alamos, New Mexico; Oak Ridge, Tennessee and Hanford, Washington.
Albert Einstein did not work directly on the project, but signed the 1939 letter to Franklin D. Roosevelt suggesting that the United States develop nuclear weapons before the Germans and outlining ways to help American scientists. Hungarian physicists Leo Szilard, Eugene Wigner and Edward Teller authored the letter.
Although it was a secret project, more than 120,000 people worked on the project in some capacity but were unaware of the project's nature until after the first bomb was dropped on Japan. Harry Truman became aware of the project when he became president; as vice-president, he had known nothing.