Manhattan Project

Manhattan Project, the U.S. effort in World War II that developed the atomic bomb. The possibility of developing an atomic bomb became evident late in 1938 when scientists in Germany successfully split a uranium atom by bombarding it with neutrons. In the United States, Leo Szilard, a physicist at the University of Chicago, recognized that as a result of such nuclear fission, a critical mass of uranium could produce enough neutrons to generate a chain reaction of radioactive material culminating in an enormous nuclear explosion. Prodded by Szilard, Albert Einstein, world‐renowned German physicist who had fled to the United States, wrote to President Franklin D. Roosevelt on 2 August 1939 warning that the Nazis might develop an atomic bomb.

Roosevelt formed a committee of scientists headed first by Enrico Fermi and subsequently by Vannevar Bush (renamed the National Defense Research Committee) to study the feasibility of building such a weapon. In October 1941, this was merged into the new Office of Scientific Research and Development. In spring 1942, Ernest Lawrence of the University of California, Berkeley, demonstrated that in addition to the scarce uranium isotope U‐235, the more available U‐238 could be converted into a new element, plutonium, which was also fissionable. After the United States entered the war, Roosevelt gave the development of nuclear weapons top priority, and in August 1942 he assigned the top‐secret project to the U.S. Army Corps of Engineers. Its code name, the “Manhattan Project,” derived from the Manhattan Engineer District established to supervise the weapon's construction. The commanding officer, Maj. (later Brig. Gen.) Leslie R. Groves, spent $2 billion to develop the atomic bomb.

The Manhattan Project had four main facilities. In the basement of the unused football stadium of the University of Chicago, scientists Enrico Fermi and Arthur Compton built an atomic pile and in December 1942 produced the first chain reaction in uranium. At Hanford, Washington, a plant produced plutonium‐239 from uranium‐238. The Clinton Engineer Works at Oak Ridge, Tennessee, separated uranium‐235 from uranium‐238 through gaseous diffusion. A secret new laboratory, headed by physicist J. Robert Oppenheimer, was built in 1943 on a secluded mesa at Los Alamos, New Mexico, to design and build atomic bombs.

Secrecy was an obsession with Groves, and only a handful of the 125,000 people at the Project's four facilities understood the purpose of their work. Just a few military and congressional leaders knew the reason for the project's huge expenditures, which were concealed within War Department appropriations.

Since scientists in Britain had been working toward a bomb since 1940 and discovered the new element called “plutonium,” Roosevelt and British prime minister Winston S. Churchill cooperated in the research. However, in September 1944, the two leaders decided not to share their information with the Soviet Union. Russia initiated an intense espionage effort in Britain and the United States to aid its own program, headed by physicist Igor Kurchatov.

Soviet leader Josef Stalin learned details of the bomb's progress from Communist sympathizers, among them atomic scientist Klaus Fuchs in Britain, and David Greenglass, an American soldier stationed near Los Alamos. In a controversial trial in 1950, following Fuchs's postwar confession, Greenglass testified that his brother‐in‐law, Julius Rosenberg, and Rosenberg's wife, Ethel, had passed to the Russians atomic secrets he had obtained. The Rosenbergs were executed in 1953. (The Nazi regime did not race to build an atomic bomb, although whether this was due to pessimistic miscalculations by its leading physicist, Werner Heisenberg, or to his moral opposition to such a weapon, remains unclear.)

Following Roosevelt's death on 12 April 1945, President Harry S. Truman was told about the atomic bomb (code‐named “S‐1”) twelve days later. With Germany nearing surrender and the construction of a test device only three months away, Truman created an Interim Committee to study the use of atomic bombs against Japan.

On 31 May 1945, the Interim Committee, composed of Secretary of War Henry L. Stimson, Secretary of State designate James Byrnes, Harvard president James Conant, physicist and educator Karl Compton, Vannevar Bush, and a few others, listened to Oppenheimer predict the bomb would be equal to 2,000 to 20,000 tons of TNT and with its blast and radiation would kill perhaps 20,000 Japanese. After consulting other scientists and the Joint Chiefs of Staff, the committee agreed on 1 June 1945 that for maximum psychological effect, the atomic bomb should be used without warning against a Japanese city containing a military facility.

Not all the scientists working on the Manhattan Project agreed with this. Szilard, James Franck, and a majority of the scientists at the Chicago laboratory asserted that military use against a Japanese city was unnecessary and immoral and would start a postwar nuclear arms race. In response to their petition for a test demonstration and warning for Japan, a special scientific advisory committee—composed of Fermi, Lawrence, Oppenheimer, and Arthur Compton—met on 16 June but rejected the idea of a noncombat demonstration (the bomb might not explode, and even if it did, its lethality would not be adequately demonstrated).

On 16 July 1945, the first atomic weapon test, code‐named “Trinity,” was held on a desert bombing range at Alamogordo, New Mexico, 200 miles south of Los Alamos. Mounted on a metal tower, the test device—13.5 pounds of plutonium inside 2.5 tons of explosives—was exploded at 5:29 A.M. as Groves, Oppenheimer, Bush, and others watched in awe. The blast equaled 15,000–20,000 tons of TNT and generated a fireball visible for 60 miles.

Truman learned of the successful test while at the Potsdam Conference in Germany. After mentioning cryptically to Stalin that the United States had a new weapon, Truman on 24 July ordered preparations for use against Japan. On the 26th, he issued the Potsdam Declaration, a vague modification of unconditional surrender. When Tokyo declined to consider the offer because it did not guarantee retention of the emperor, Truman, on 30 July, ordered the Army Air Forces to use America's two atomic bombs—one uranium‐cored, the other plutonium‐cored—against Japan. On 6 and 9 August, solitary American B‐29s carried out the atomic bombings of Hiroshima and Nagasaki. The bombings, combined with the Soviet Union's declaration of war against Japan on 8 August, led Tokyo to surrender on 14 August 1945. World War II ended; the atomic age had begun.
[See also Atomic Scientists; Nuclear Weapons; Science, Technology, War, and the Military; World War II: Military and Diplomatic Course; World War II: Domestic Course; World War II: Postwar Impact; World War II: Changing Interpretations.]

Bibliography

Martin Sherwin , A World Destroyed: Hiroshima and the Origins of the Arms Race, 1973; rev. ed. 1987.
Leslie R. Groves , Now It Can Be Told: The Story of the Manhattan Project, 1975.
Richard Rhodes , The Making of the Atomic Bomb, 1986.
James G. Hershberg , James B. Conant: Harvard to Hiroshima and the Making of the Nuclear Age, 1993.
Gar Alperovitz , The Decision to Use the Atomic Bomb—and the Architecture of an American Myth, 1995.
Barton J. Bernstein , The Atomic Bombings Reconsidered, Foreign Affairs (January–February 1995), pp. 135–52.
Robert P. Newman , Truman and the Hiroshima Cult, 1995.
Dennis D. Wainstock , The Decision to Drop the Atomic Bomb, 1996.

Dennis D. Wainstock