May 3 - German troops occupy Norway, seizing the world's only heavy-water production plant at Vemork. May 19 - Frisch and Rudolf Peierls submit a memorandum to the British government estimating the critical mass of uranium 235 needed for an atomic bomb and urging a bomb research project. June 3 - German scientists fail to observe neutron multiplication in the reactor in Hamburg. June 15 - Using the Berkeley cyclotron, Philip Abelson and Edwin McMillan demonstrate that neutrons captured by uranium 238 lead to the creation of elements 93 and 94, neptunium and plutonium.
1941
January - Based on experiments with a natural uranium reactor, the Germans reject graphite as a moderator. February 23 - Glenn Seaborg discovers new element (atomic number 94), which they call plutonium. March 28 - American physicists confirm that plutonium is fissionable, thus usable for a bomb. July - British 'MAUD' Committee reports that a weapon could be made with 22 pounds of pure uranium 235. December 6 - President Roosevelt authorizes the Manhattan Engineering District. The secret U.S. project to build an atomic bomb, later to be called the Manhattan Project, is put under the direction of the Office of Scientific Research and Development.
1942
September 23 - Brigadier General Leslie Groves is put in charge of the Manhattan Project. He recruits J. Robert Oppenheimer as Scientific Director. November 5 - Construction of a uranium isotope separation plant begins at Oak Ridge, Tennessee. December 2 - Enrico Fermi at the University of Chicago produces the world's first controlled and sustained nuclear fission reaction.
1943
January - Planning begins for construction of reactors at Hanford, Washington, to breed plutonium for a bomb. March 15 - Oppenheimer moves the bomb development to a secret laboratory located at Los Alamos, New Mexico.
1944
August 26 - Bohr presents his memorandum on intentional control of nuclear weapons to Roosevelt. November - First batch of spent fuel obtained from Hanford reactors. November - The ALSOS mission obtains documents implying that the German's rate of progress toward a bomb had diminished.
1945
January - First plutonium reprocessing begins at Hanford. January 20 - First uranium 235 separated at Oak Ridge, Tennessee. June 11 - The Franck Report is sent to Secretary of War Stimson. July 16 - U.S. explodes the world's first atomic bomb, the Trinity test, at Alamogordo, New Mexico. August 6 - Little Boy, an uranium bomb, was dropped on Hiroshima, Japan. Between 80,000 - 140,000 people are killed. August 9 - Fat Man, a plutonium bomb, was dropped on Nagasaki, Japan. About 74,000 people are killed.
1946
June 14 - Bernard Baruch presents the Acheson-Lilienthal plan to internationalize the atomic energy. It is rejected by the U.S.S.R. June 30 - First subsurface detonation by U.S. is achieved at Bikini Atoll in the Pacific. July - Demonstrations are held in Times Square, New York, against nuclear testing. August 1 - President Harry S. Truman signs the Atomic Energy Act. Its purpose is to control the development and production of nuclear weapons and to direct the research and development of peaceful uses of nuclear energy. December 25 - The Soviet Union achieves its first nuclear chain reaction in Moscow. December 31 - Atomic Energy Commission (AEC) takes over nuclear weapons program from the Army.
1947
January - The United Kingdom authorizes the development of nuclear weapons. June - Under the direction of William Penney, the UK begins design of its plutonium bomb. August - The United Kingdom builds its first atomic reactor.
1948
April, May - U.S. conducts atomic tests at Eniwetok Atoll.
1949
April 4 - NATO established. August 29 - Soviet Union detonates its first atomic bomb, Joe 1, at Semipalatinsk in Kazakhstan. It is a copy of the Fat Man bomb and has a yield of 21 kilotons. October 30 - General Advisory Committee of the AEC recommends that the more powerful atomic bombs should be built rather than hydrogen bombs.