By the summer of 1945, World War II had raged across several continents for six years, beginning when German forces invaded Poland in 1939.

To a world grown weary of death and destruction, the surrender of Nazi Germany on May 8, 1945, was welcome news. Japan, however, vowed to fight to the very end. Their resolve was evident in the Battle of Okinawa: By the time it ended in June, Allied forces lost more than 14,000 soldiers and Japan's military suffered more than 77,000 deaths — plus the loss of an estimated 100,000 Japanese civilians.

Against this backdrop, Allied forces drew up plans for Operation Downfall, a large-scale invasion of Japan. But with Allied casualties from an invasion estimated to reach 1 million deaths — plus another 10 million Japanese casualties, including civilians — Allied planners searched for another way to end the war, which they found in the Manhattan Project.

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This is a mockup of "Little Boy," the atomic bomb that was dropped over Hiroshima, Japan, on Aug. 6, 1945.

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This is a mockup of "Little Boy," the atomic bomb that was dropped over Hiroshima, Japan, on Aug. 6, 1945.

Credit: U.S. National Archives The Manhattan Project

Concerned over reports that Nazi Germany was developing a new type of weapon using radioactive uranium, in 1939 U.S. government officials began to investigate the potential of uranium — particularly the uranium-235 isotope — for making a powerful bomb.

Within a matter of months, a coalition of American, British and European scientists — many of whom were refugees from Germany, Italy and other fascist nations — began collaborating on a vast international project to develop a uranium-based bomb before any Axis powers beat them to it.

Because one early component of the project was based in the U.S. Army's Manhattan District, the name Manhattan Project