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The Manhattan Project

Written by: Edward G. Lengel, The National World War II Museum

By the end of this section, you will:

  • Explain the causes and effects of the victory of the United States and its allies over the Axis Powers

Suggested Sequencing

Use this narrative with the Dropping the Atomic Bomb Decision Point and the Was the Use of the Atomic Bomb Justified? DBQ Lesson to show the development of the United States’ nuclear program and subsequent use in the Hiroshima and Nagasaki bombings.

Fundamental discoveries about the nature of the atom took place during the most war-torn century in human history. By the 1920s and 1930s, scientists were intensively studying the military ramifications of atomic power. In 1938, German chemist Otto Hahn scored a breakthrough by not only splitting the uranium atom but also discovering the immense explosive potential of the process. He and other German scientists immediately moved on to focus their research on creating an atomic bomb for the Nazi state.

Scientists in other nations quickly became aware of the German work in this field and initiated atomic programs of their own. Nuclear research in Britain, led by German scientists who had fled the Nazi regime, surged ahead with the discovery that it would be possible to build a bomb with only small quantities of the rare isotope uranium-235. Lacking this knowledge, and assuming it would take many years to acquire the supplies necessary to build a bomb, German scientists had slowed their work by the early 1940s. But other scientists did not know this. On August 2, 1939, famed scientist Albert Einstein wrote to President Franklin Roosevelt urging him to accelerate his country’s atomic program to ensure that the Germans did not develop the bomb first.

An alarmed Roosevelt responded energetically, especially after the Japanese attack on Pearl Harbor and the American entry into World War II in Europe and Asia. Led by his scientific advisors to believe that, with great effort, an atomic weapon could be developed by 1944, on June 17, 1942, the president initiated the atomic program that came to be called the Manhattan Project. Unlike the Germans, who assumed they would win the war quickly and that continuing their atomic program was thus not worth the trouble, the Americans and British anticipated a long conflict and so were deeply committed to their projects. They shared information with each other along the way, but not with the Soviet Union. Soviet espionage nevertheless monitored the Anglo-American programs with a degree of success that was not known until many years later.

The Manhattan Project, named after a supervisory district of the U.S. Army Corps of Engineers in Manhattan, New York, oversaw the U.S. atomic program. It was headed by General Leslie R. Groves and carried out its work at facilities in Illinois, Tennessee, Washington state, and New Mexico ([link]Figure_12_03_ManProjMap[/link]). Progress was rapid, thanks not just to scientific work but to America’s vast industrial capacity. In December 1942, scientists Enrico Fermi and Arthur Compton created the first-ever uranium chain reaction in the basement of the University of Chicago’s football stadium. In a facility built the following year on a mesa at Los Alamos, New Mexico, meanwhile, scientist J. Robert Oppenheimer and his team worked to create the first atomic bomb.

Figure a is a portrait of Enrico Fermi. Figure b is a map of the United States with a few cities emphasized: Radiation Laboratory in Berkeley, California, Project Camel in Inyokern, California, Hanford Engineer Works in Richland, Washington, Vanadium Corporation in Monticello, Utah, Project Alberta in Wendover, Utah, US Vanadium Corporation in Uravan, Colorado, Los Alamos Laboratory—Project Y in Los Alamos, New Mexico, Project Trinity in Alamogordo, New Mexico, Project Ames in Ames, Iowa, Metallurgical Laboratory in Chicago, Illinois, Health Project in Rochester, New York, Manhattan District Headquarters and Clinton Engineering Works in Oak Ridge, Tennessee, Sylacauga, Alabama, and Washington DC.

(a) Enrico Fermi, one of the Manhattan Project scientists, created the first nuclear reactor, the Chicago Pile-1. (b) Important sites associated with the Manhattan Project were scattered across the country.

The expenses of the top-secret Manhattan Project were concealed from Congress, subsumed in appropriations for the War Department. By the time the war ended, they totaled approximately $2 billion, dwarfing every other wartime military project except the creation of the B-29 Superfortress bomber. Roosevelt ensured that his atomic scientists were never short of funds, however, knowing that if the project succeeded, no one would question the cost.

The first bomb was nearly complete at the time of President Roosevelt’s death on April 12, 1945. New president Harry S. Truman ordered the program to move forward despite Germany’s impending surrender, with a view toward possibly using the weapon against Japan. While the interim committee Truman created considered the military, political, and moral advisability of using the bomb, Oppenheimer’s team completed the first-ever atomic weapon and prepared it for testing.

The test, codenamed “Trinity,” took place on July 16, 1945, in the desert at Alamogordo, New Mexico, 200 miles south of Los Alamos. The device, mounted on a metal tower, consisted of just 13.5 pounds of plutonium encased in two-and-a-half tons of explosives. It exploded at 5:29 a.m. to devastating effect, equal to the detonation of almost 20,000 tons of TNT. Groves and Oppenheimer witnessed the atomic fireball expand into a mushroom cloud visible 60 miles away. Horrified by what he saw, Oppenheimer called to mind words from the Bhagavad Gita: “Now I am become Death, the destroyer of worlds.” But it was too late to turn back. The world had entered the nuclear age.

Figure a shows a mushroom cloud lighting up the darkness. Figure b shows J. Robert Oppenheimer and General Leslie Groves standing in a wasteland.

(a) In 1945, the mushroom cloud from the first atomic weapon test, “Trinity,” could be seen as far as 60 miles away. (b) J. Robert Oppenheimer and General Leslie Groves inspect the aftermath of the explosion at Alamogordo, New Mexico, in July 1945.

On August 6, the Enola Gay, a B-29 Superfortress, dropped the uranium bomb nicknamed Little Boy, which exploded with the force of 12,500 tons of TNT 1,900 feet above the Japanese city of Hiroshima. With a blinding flash and rising mushroom cloud, the blast and resulting firestorm obliterated the city and destroyed 70,000 buildings. People were vaporized from the blast and their shadows imprinted on walls. An estimated 70,000 to 80,000 civilians and soldiers were immediately killed, and thousands later died of radiation poisoning and burns. Tormented survivors were disfigured with hanging skin and burns. President Truman sent public messages announcing the dropping of an atomic bomb and threatened more if Japan refused to surrender. Still, the Japanese government fought on.

On August 9, another B-29 bomber dropped a plutonium bomb called Fat Man on Nagasaki, with an even larger blast equivalent to 22,000 tons of TNT. Due to significant cloud cover this second bomb missed its target by a wide margin, somewhat limiting its destructive impact. Nevertheless, it killed at least 30,000 people and caused suffering for thousands of survivors. Over the next five days, conventional bombings of other major cities killed an additional 15,000 Japanese. Finally, on August 14, Japan surrendered and World War II ended.

The development of the atomic bomb and the ensuing arms race between the United States and the Soviet Union, along with their allies, ushered in the nuclear age and imperiled all humanity. Although the only atomic bombs ever used were those dropped on Hiroshima and Nagasaki during World War II, the Cold War led to the credible threat of their additional use and the fear of widespread destruction.

Review Questions

1. The costs associated with the Manhattan Project did not lead to protest primarily because

  1. Congress considered the costs to be justified
  2. President Roosevelt overruled any objections
  3. atomic bombs were inexpensive to build
  4. they were concealed from Congress

2. The atomic explosion at Alamogordo, New Mexico, on July 16, 1945

  1. failed to produce the desired results
  2. ended World War II
  3. proved the success of the Manhattan Project
  4. convinced President Truman the atomic bomb was too powerful to use

3. Development of the atomic bomb in the United States during the 1940s occurred

  1. at various locations throughout the United States
  2. primarily in Manhattan, New York
  3. exclusively under civilian leadership
  4. primarily using the research generated by American scientists

4. The American scientist who oversaw the Manhattan Project was

  1. Leslie Groves
  2. Robert Oppenheimer
  3. Enrico Fermi
  4. Otto Hahn

5. The scientific advances behind the Manhattan Project primarily benefited from

  1. the work of émigré scientists from totalitarian regimes
  2. Anglo-American military cooperation
  3. American anticipation of a short military conflict
  4. exchange of nuclear scientific knowledge between the United States and the Soviet Union

Free Response Questions

  1. Analyze the factors that led the United States to build the first atomic bomb.
  2. Describe the organization of the Manhattan Project.

AP Practice Questions


Some recent work by E. Fermi and L. Szilard . . . leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. . . . I believe therefore that it is my duty to bring to your attention the following facts and recommendations.

. . . It may be possible to set up a nuclear chain reaction in a large mass of uranium . . .

This new phenomenon would also lead to the construction of bombs . . . A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove too heavy for transportation by air. . . .

In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America.”

Albert Einstein, Letter to President Franklin Roosevelt, August 2, 1939

Refer to the excerpt provided.

1. The sentiments expressed in the excerpt most directly led to the

  1. creation of the Manhattan Project
  2. implementation of the island-hopping strategy
  3. D-Day invasion
  4. defeat of Nazi Germany

2. Which group would most likely support the argument made in the excerpt?

  1. Critics of the military-industrial complex
  2. Opponents of the Treaty of Versailles
  3. Proponents of the Lend-Lease Act
  4. Isolationists such as the America First group

3. This excerpt was written in response to the

  1. federal programs created by the New Deal
  2. rise of fascism in Europe
  3. debates about the morality of using atomic weapons
  4. expansion of communist ideology in Southeast Asia

Primary Sources

Einstein, Albert. 1939 letter to President Roosevelt.

“Trinity Test Eyewitnesses.” Atomic Heritage Foundation.

Suggested Resources

Bird, Kai, and Martin J. Sherwin. American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer. New York: Knopf, 2005.

Chambers, John Whiteclay, ed. The Oxford Companion to American Military History. Oxford, UK: Oxford University Press, 1999.

Conant, Jennet. 109 East Palace: Robert Oppenheimer and the Secret City of Los Alamos. New York: Simon and Schuster, 2005.

Kelly, Cynthia C., ed. Manhattan Project: The Birth of the Atomic Bomb in the Words of its Creators, Eyewitnesses, and Historians. New York: Black Dog & Leventhal, 2009.

Kunetka, James. The General and the Genius: Groves and Oppenheimer – The Unlikely Partnership that Built the Atom Bomb. New York: Regnery, 2015.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1987.

Weinberg, Gerhard L. A World at Arms: A Global History of World War II. Cambridge, UK: Cambridge University Press, 1994.

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