Manhattan Project: A Detailed Summary

The Manhattan Project was the codenameA secret name used to hide the real identity or purpose of a military operation. for the secret American program to create the first atomic weapon during the end of World War II.  The project was run by General Leslie Groves, and its scientific work was led by Robert Oppenheimer, an American physicist who was the head of the Los Alamos Laboratory in New Mexico, where the first atomic bombs were designed. Eventually, the Manhattan Project resulted in the atomic bombings of Hiroshima and Nagasaki in Japan, to end World War II. Furthermore, the creation of the atomic bomb had a profound impact on the Cold War, which unfolded in the decades after the Second World War.

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J. Robert Oppenheimer

At a Glance

BornApril 22nd, 1904, in New York City, New York

DiedFebruary 18th, 1967, in Princeton, New Jersey

RoleScientific Director, Manhattan Project

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Early Education

Oppenheimer studied chemistry at Harvard University and earned his PhD in physics from the University of Göttingen, Germany, in 1927.

Los Alamos

In 1943, Oppenheimer became director of the newly built Los Alamos Laboratory in New Mexico, where scientists worked to design the first atomic bomb.

Trinity Test

On July 16th, 1945, the first atomic bomb was tested at the Trinity site in the New Mexico desert, 210 miles (338 kilometers) south of Los Alamos.

Hiroshima and Nagasaki

The bombs dropped on Hiroshima on August 6th, 1945, and on Nagasaki on August 9th, 1945, used designs developed under Oppenheimer's leadership at Los Alamos.

Security Hearing

In 1954, the U.S. Atomic Energy Commission revoked Oppenheimer's security clearance after a hearing that examined his past political associations, ending his official government work.

Fermi Award

In December 1963, President Lyndon B. Johnson presented Oppenheimer with the Enrico Fermi Award, partly as a public acknowledgment of his scientific contributions.

Source: National Archives© factsthatcrunch.comMore World War II Facts →

Manhattan Project – Causes

The Manhattan Project had its beginnings in 1939 after it was determined that Nazi German scientists had discovered nuclear fissionThe splitting of an atom’s nucleus, releasing a massive amount of energy that can power a weapon. in 1938.  Nuclear fission, it was feared, would allow Germany to begin the process of constructing an atomic bomb.  These fears caused physicists Leó Szilárd and Albert Einstein to draft a letter in August of 1939 warning the United States of the consequences of Nazi Germany having access to atomic weaponry. The letter was delivered to United States President Franklin D. Roosevelt and urged him to undertake the process of having the United States stockpile large amounts of uranium oreA naturally occurring rock that contains uranium, a material used as fuel for atomic bombs., which could be used as a source for possible atomic bombs.  The hope was that the United States could develop an atomic weapon before Nazi Germany.

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How the 1939 Einstein-Szilard Letter Led the United States Toward an Atomic Bomb

1

German scientists discover nuclear fission in 1938, showing that splitting an atom could release enormous energy and possibly power a weapon.

2

Physicists Leo Szilard and Albert Einstein fear that Nazi Germany will use this discovery to build an atomic bomb before other countries can act.

3

In August 1939, Szilard and Einstein draft a letter to President Franklin D. Roosevelt warning him of this danger and urging the United States to stockpile uranium ore.

4

Roosevelt receives the letter and authorizes early federal support for uranium research, laying the groundwork for a formal American atomic program.

5

This chain of concern leads directly to the launch of the Manhattan Project in 1942, placing the United States on a path to building the first atomic bomb.

The discovery of nuclear fission in 1938 set off a chain of urgent decisions that moved the United States from awareness to action. The Einstein-Szilard letter of August 1939 was the key event that connected scientific alarm to presidential policy.

Source: Library of Congress© factsthatcrunch.comMore World War II Facts →

Manhattan Project – Summary

The Manhattan Project gained momentum in 1942 under the leadership of General Leslie Groves and scientific director Robert Oppenheimer. Furthermore, it brought together some of the world’s most brilliant scientists, including physicists such as: Enrico Fermi, Niels Bohr, and Richard Feynman. American scientists began the difficult work of organizing the secret program.  Initially, much of the research and scientific work was completed at colleges throughout the United States. More specifically, the project’s research and production facilities were spread across multiple secret sites in the United States, including Los Alamos, New Mexico (where the bombs were designed and assembled), Oak Ridge, Tennessee (uranium enrichment), and Hanford, Washington (plutonium production). Scientists worked in secrecy to understand the processes of nuclear fission and to develop the weapons using enriched uranium-235A processed form of uranium prepared so it can sustain the chain reaction needed for an atomic bomb. and plutonium-239A radioactive element produced artificially and used as an alternative explosive material in atomic bombs.. This research continued until the first ever atomic test in 1945.

The first ever test of an atomic weapon was codenamed ‘Trinity’ and took place in the desert of New Mexico near Alamogordo on July 16th, 1945.  The site was chosen for its remote nature so as to limit any impact on civilian populations.  At the Trinity Site, a platform was created in which a tower was constructed that would hold the weapon component of the first atomic test.  The device was called the ‘gadget’ and when exploded it was considered to be a full-scale atomic test.  The explosion was equivalent to approximately 20 kilotonsA unit measuring explosive force equal to a thousand tons of TNT, used to describe nuclear blast size. of TNT.  The United States had entered the atomic age and set the stage for the end of World War II and the start of the Cold War.

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first chain reaction!Nobel Prize, 1938know this person!

Enrico Fermi

in World War II

Italian American physicist on the Manhattan Project

Early Career: Fermi won the Nobel Prize in Physics in 1938 and moved from Italy to the United States that same year.

Chicago Pile-1: On December 2nd, 1942, Fermi led the University of Chicago team that achieved the world's first controlled nuclear chain reaction.

Los Alamos: He later worked at the Los Alamos Laboratory in New Mexico, where the atomic bombs were designed and assembled.

Source: U.S. Department of Energy© factsthatcrunch.comMore World War II Facts →

Manhattan Project – Significance

Less than a month after the Trinity Site test, the United States dropped two atomic bombs on Hiroshima and Nagasaki, Japan. These two atomic bombings, along with the Soviet Union’s declaration of war on Japan, led Japan to surrender, which effectively ended World War II. As such, historians consider the Manhattan Project to be a significant aspect of World War II and its ultimate end. Furthermore, the two atomic bombings in Japan set the stage for the ‘atomic age’ and the tensions of the latter half of the 20th century. This is because the Manhattan Project had profound historical significance.

For instance, it demonstrated the destructive power of nuclear weapons, led to the nuclear arms raceA competition between countries to build more and more powerful nuclear weapons than their rivals. of the Cold War, and sparked ethical debates about the ethics surrounding the use of atomic weapons. For instance, historians have long debated the use of the atomic bombs in Japan, and questioned the validity of their use. Some historians have argued in favor of their use, while others have argued against it. Click on the links to read more detailed information on the reasons for and against the use of the atomic bombs against Japan.

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This colorized photograph shows General Leslie Groves, the military director of the Manhattan Project, alongside a map of the Far East including Japan. Groves took command of the project in 1942 and oversaw the military planning that connected the bomb program to its targets in the Pacific.

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To cite this article as a source, use one of the formats below.

MLA: Beck, Elias. “Manhattan Project: A Detailed Summary.” HistoryCrunch, 2 October 2026, https://historycrunch.com/manhattan-project/.

APA: Beck, E. (2026). Manhattan Project: A Detailed Summary. HistoryCrunch. https://historycrunch.com/manhattan-project/

Chicago: Beck, Elias. “Manhattan Project: A Detailed Summary.” HistoryCrunch. October 2, 2026. https://historycrunch.com/manhattan-project/

Sources

• The National WWII Museum
• Gerhard L. Weinberg, A World at Arms.
• Max Hastings, Inferno / All Hell Let Loose.