Hiroshima and Nagasaki 

On August 6, 1945, during World War II (1939-45), an American B-29 bomber dropped the world’s first deployed atomic bomb over the Japanese city of Hiroshima. The explosion wiped out 90 percent of the city and immediately killed 80,000 people; tens of thousands more would later die of radiation exposure. Three days later, a second B-29 dropped another A-bomb on Nagasaki, killing an estimated 40,000 people. Japan’s Emperor Hirohito announced his country’s unconditional surrender in World War II in a radio address on August 15, citing the devastating power of “a new and most cruel bomb.

  The Manhattan Project

Even before the outbreak of war in 1939, a group of American scientists–many of them refugees from fascist regimes in Europe–became concerned with nuclear weapons research being conducted in Nazi Germany. In 1940, the U.S. government began funding its own atomic weapons development program, which came under the joint responsibility of the Office of Scientific Research and Development and the War Department after the U.S. entry into World War II. The U.S. Army Corps of Engineers was tasked with spearheading the construction of the vast facilities necessary for the top-secret program, codenamed “The Manhattan Project ” (for the engineering corps’ Manhattan district).

 After World War II, most of Hiroshima would be rebuilt, though one destroyed section was set aside as a reminder of the effects of the atomic bomb. Each August 6, thousands of people gather at Peace Memorial Park to join in interfaith religious services commemorating the anniversary of the bombing.
 Over the next several years, the program’s scientists worked on producing the key materials for nuclear fission–uranium-235 and plutonium (Pu-239). They sent them to Los Alamos, New Mexico, where a team led by J. Robert Oppenheimer worked to turn these materials into a workable atomic bomb. Early on the morning of July 16, 1945, the Manhattan Project held its first successful test of an atomic device–a plutonium bomb–at the Trinity test site at Alamogordo, New Mexico.