The United States detonates the world's first atomic bomb at a test site in New Mexico on July 16, 1945. Less than a month later, atomic bombs were dropped on the Japanese cities of Hiroshima and Nagasaki. The devastation led to Japan's unconditional surrender and brought an end to World War II.
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In 1939, physicists Albert Einstein, left, and Leo Szilard drafted a letter to U.S. President Franklin D. Roosevelt, urging him to research atomic bombs before the Germans could build one first. By 1942, the United States had approved the top-secret Manhattan Project to build a nuclear reactor and assemble an atomic bomb.
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In 1942, U.S. Army Col. Leslie R. Groves, left, was appointed to head the Manhattan Project. On the right is physicist J. Robert Oppenheimer, who led the Los Alamos National Laboratory in New Mexico.
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Los Alamos workers pose on a platform stacked with 100 tons of TNT. It was to be used to gauge radioactive fallout.
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The Manhattan Project also involved research facilities in Oak Ridge, Tennessee, and Hanford, Washington. Billboards, like this one in Oak Ridge, reminded workers of the project's top-secret nature.