Officially lasting from June 18, 1942 to August 25, 1947, the Manhattan Project employed over 130,000 people throughout the United States. The top-secret project to develop the atomic bomb forever altered the world we live in. Access the timeline by year to learn about key events that shaped this revolutionary and controversial undertaking.
December, 1938: German physicists Otto Hahn and Fritz Strassman discover nuclear fission
August 2, 1939: President Franklin Delano Roosevelt receives the “Einstein Letter” warning of Nazi Germany’s efforts to create an atomic weapon
September 1, 1939: Nazi Germany invades Poland, World War II begins
October 21, 1939: The first meeting of the Advisory Committee on Uranium is held in Washington, DC
June 27, 1940: The National Defense Research Committee (NDRC) is created to organize US scientific resources for war including research on the atom and the fission of uranium
February 24, 1941: Physicist Glenn Seaborg discovers plutonium
December 7, 1941: Japan attacks a US military installation at Pearl Harbor, HI
December 8, 1941: The United States declares war on Japan
December 11, 1941: The United States declares war on Germany and Italy (Germany and Italy had declared war on the United States first)
January 19, 1942: President Roosevelt approves the creation of an atomic bomb. Arthur Compton creates the Metallurgical Laboratory (Met Lab) at the University of Chicago
June 18, 1942: The Army Corps of Engineers Manhattan District is formally established
September 17, 1942: US Army Brigadier General Leslie Groves (still a Colonel until Sept. 23) is appointed head of the Manhattan Engineer District
September 19, 1942: Groves approves Oak Ridge, TN as a secret site for uranium enrichment
November 16, 1942: The Chicago Pile (CP-1), the world’s first experimental nuclear reactor, begins construction at the University of Chicago
November 25, 1942: Groves approves Los Alamos, NM as a secret site for atomic bomb development and construction
December 2, 1942: CP-1 reaches criticality
January 16, 1943: Groves approves Hanford, WA as a secret site for plutonium production
November 4, 1943: Oak Ridge’s X-10 Graphite Reactor, the world’s first full-scale reactor, reaches criticality
June 6, 1944: Allied forces gain a foothold on the European continent in northern France, more commonly known as D Day
September 26, 1944: Hanford’s B Reactor reaches criticality. It would reach full plutonium production power in February, 1945
April 12, 1945: President Roosevelt dies. His successor Harry Truman first learns of the Manhattan Project’s existence the next day
May 7, 1945: Nazi Germany surrenders to allied forces
July 16, 1945: The Gadget, the world’s first atomic test device, successfully detonates at the Trinity Site in the remote New Mexico desert
July 26, 1945: Truman issues the Potsdam Declaration, warning Japan of incomprehensible destruction if they do not surrender
August 6, 1945: The Little Boy atomic bomb, designed in Los Alamos and fueled by enriched uranium from Oak Ridge, detonates over Hiroshima, Japan, killing and wounding tens of thousands
August 9, 1945: The Fat Man atomic bomb, designed in Los Alamos and fueled by plutonium from Hanford, detonates over Nagasaki, Japan, killing and wounding tens of thousands
August 14, 1945: Japan announces their plan to surrender
September 2, 1945: The Japanese formally signed the Instrument of Surrender on September 2, 1945, officially ending the most deadly and destructive war in human history
August 1, 1946: President Truman signs the Atomic Energy Act, establishing the Atomic Energy Commission that transfers the United States’ nuclear program from military to civilian control
January 1, 1947: The Atomic Energy Commission assumes control of the Manhattan Engineer District
August 25, 1947: The Manhattan Engineer District is officially abolished, formally ending the Manhattan Project
