Manhattan Project (1942–1945)

Code name of the U.S. military–civilian mission to produce an atomic bomb during World War II. The top-secret Manhattan Project involved the labor of 125,000 people and cost the United States about $2.2 billion.

In August 1939, on the eve of World War II, U.S. and British physicists had evidence that the energy released from nuclear fission could be used to produce explosive weapons. That same year, physicists Albert Einstein and Leo Szilard, immigrants to the United States from Europe, informed President Franklin D. Roosevelt of the military possibilities of nuclear fission.

In October 1941, when President Roosevelt ordered the U.S. government to fund atomic bomb research, the Japanese surprise attack on Pearl Harbor was two months in the future. Roosevelt approved the funding while under the assumption that such weapons were needed to compete with the Nazis, whom intelligence sources indicated were well on the way to creating their own atomic weapons.

The attack on Pearl Harbor and the U.S. entry into World War II accelerated U.S. plans to proceed with atomic weapon production. The Manhattan Project, or Manhattan Engineer District, was officially instituted in June 1942 under the jurisdiction of the Army Corps of Engineers. In September of that year, U.S. General Leslie Groves became the director of the Manhattan Project. His first task was to order the construction of sites for the manufacture of uranium and plutonium for the bombs. In 1943, uranium production [Page 442]was scheduled to begin in Oak Ridge, Tennessee, and plutonium production in Hanford, Washington. In December 1942, Italian physicist Enrico Fermi and his colleagues produced the first nuclear chain reaction at the University of Chicago, a development that was essential to the manufacture of an atomic bomb.

Theoretical physicist J. Robert Oppenheimer was appointed to direct the actual bomb laboratory in Los Alamos, New Mexico. In 1943, 100 of the best minds in physics and engineering in the United States were recruited and gathered under the strictest secrecy to create the atomic bomb, then nicknamed “the gadget.” In the months to come, thousands more scientists, engineers, technicians, workers, and their families would arrive in Los Alamos, a secluded mesa north of Santa Fe, New Mexico. In all, some 5,000 men and women worked at the Los Alamos Scientific Laboratory during World War II.

At first, the scientists were oriented toward constructing what were called gun-type bombs, in which fissile material was shot together to create the necessary chain reaction. The scientists faced a setback, however, when they learned that the gun method would work only for a uranium bomb. The scientists wanted to try using implosion for the plutonium bomb, in which nuclear material is compressed inward to create a nuclear explosion. Only after months of experimentation and struggle were Los Alamos scientists and engineers able to create the implosion weapon. Although they were certain that the uranium gun-type bomb (nicknamed “Little Man”) would explode successfully without being tested, the implosion plutonium bomb (nicknamed “Fat Boy”) required a dry run.

Shortly after dawn on July 16, 1945, the implosion bomb was successfully detonated at the Trinity test site, located in an area known as Jornada del Muerto (Journey of Death) in Alamogordo, New Mexico. The bomb proved to be equivalent to 21,000 pounds of explosives. President Harry S. Truman received the news that the Trinity test was a success while at the Potsdam Conference in Germany. After conferring with military leaders, he ordered the uranium bomb to be dropped on Hiroshima, Japan, on August 6, 1945, and the second plutonium bomb to be dropped on Nagasaki on August 9, 1945.

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