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German physicits learn how to split a uranium atom, causing fears that they may create an atomic bomb.
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Leo Szilard and Eugene Wigner drafted a letter to President Franklin D. Roosevelt that was signed by Albert Einsteain, urging the U.S. to accelerate nuclear research and obtain large stores of uranium. They were worried that Nazi scientists could create an atomic bomb before the U.S. did, which would be a disaster. This letter started the U.S. nuclear program, and without it the atom bomb probably would not have happened.
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A meeting was held between Lyman Briggs, Szilard, Wigner, and Edward Teller helt a meeting to evaluate the concerns of the letter. They determined that uranium could produce a bomb vastly stronger than any other bombs. This confirmed that an atom bomb could be made
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President Roosevelt signed executive order 8807, electing Vannevar Bush as the director of the newly formed Office of Scientific Research and Development. This office researched and developed uranium.
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The U.S. atomic program was approved, and there was also a Top Policy Group to control it consisting of top politicians and military officals. This projet was led by the army, not the navy.
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The U.S declares war on Germany and Italy
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Chicago Pile-1, at the University of Chicago, goes critical under the leadership of Enrico Fermi
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The Trinity nuclear test of an implosion-style nuclear bomb is exploded at Alamogordo, New Mexico. Many of the scientists developing the bomb begin to have doubts about what they have created.
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President Truman approved atomib bomb to be used on Japan, on one of four cities.
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The uranium bomb Little Boy is detonated over Hiroshima, Japan, killing over 100,000 people.
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The second atomic bomb was dropped over Nagasaki, Japan.
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Oppenhemier refused to develop the Hydrogen Bomb, not wanting to cause any more destruction.