What was the Manhattan Project? - Definition, Scientists, & Timeline

Instructor: Mark Pearcy
In this lesson, we'll be looking at the Manhattan Project, which was the name given to the American endeavor to build the world's first nuclear bomb. After learning about this incredibly complex and destructive venture, you can test your knowledge with a quiz.

A Fateful Letter

It sounded like science fiction.

In 1939, Albert Einstein wrote a letter to President Franklin Roosevelt. In it, he described a scientific possibility which, to most people, would've sounded crazy: the possibility that within a few short years, scientists could create a single bomb that could destroy an entire city. Despite its fantastical imagery, Einstein was deadly serious, and since the world was about to be plunged into war, the nation that built such a weapon first would emerge victorious.

Six years later, almost to the day, the United States detonated a nuclear bomb over Hiroshima. The story of how this bomb was built is known best by the code name representing the entire national effort behind it: the Manhattan Project.

Origins of the Manhattan Project

The idea of nuclear fission (the process that allowed for a nuclear explosion) had been theorized by a Hungarian physicist, Leó Szilárd, in 1933. By the eve of World War II, the idea had become more or less accepted by the physics community, especially for potential as an almost limitless energy source. But the possibility that it might be used for a more destructive purpose was also there, and as war broke out in 1939, Nazi Germany began its own effort to create a nuclear weapon. It was clear that the United States needed their own initiative, and had to beat the Germans to the bomb.

The 'project' (nicknamed 'Manhattan' for the initial Army department) was a complex venture, involving thousands of personnel at multiple sites, across the nation. It's amazing that most of the people involved never really knew what they were building. The military officer in charge, General Leslie Groves, made secrecy a priority, insisting on a process of compartmentalization, in which different departments were allowed to know only about the work they were doing, and nothing else.

General Leslie Groves, overall director of the Manhattan Project

The scientific director, J. Robert Oppenheimer, was in charge of the actual creation of the bomb, while Groves was responsible for procuring the materials and personnel to get it done. The project was one of the most closely guarded secrets in the history of the United States--for example, when Harry Truman was chosen to be Franklin Roosevelt's vice-president, the first he heard about the Manhattan Project was after Roosevelt's death, only weeks before the bomb's completion.

J. Robert Oppenheimer, the scientific director of the Manhattan Project

Though the project was spread across the U.S., the major work was done at Los Alamos, New Mexico, where the bomb was designed. The site was chosen for security purposes and for privacy, since it was hundreds of miles from international borders and far from population centers. As the project grew in size, so did the installation at Los Alamos. At its peak, thousands of scientists, workers, and their families called the site home, even though it was only officially represented by a post office box number (No. 1663).

Los Alamos, New Mexico, home of the Manhattan Project
Los Alamos

The First Test

In May 1945, the project was sufficiently advanced to warrant a test, an operation codenamed 'Trinity.' an explosive model nicknamed 'the gadget' was constructed on a platform-based tower, around 30 meters tall (to approximate how it might behave when dropped from a bomber).

The platform for the Trinity nuclear test, July 1945

On July 16, 1945, the gadget was exploded.

The Trinity nuclear explosion, at 16 milliseconds after detonation

The Trinity nuclear explosion, at 15 seconds after detonation

Later, Oppenheimer claimed that that explosion made him think of words from the Hindu text, the Bhagavad-Gita: 'Now I am become Death, the destroyer of Worlds.'


At this point, the reality of the Project came home to many of the people who had been working on it since day one. Leó Szilárd, in particular, was horrified by the prospect that the weaponized version of the bomb might be used on civilian populations. The idea that this bomb could be used in this manner was deeply disturbing to Szilárd and other scientists.

In July 1945, shortly after the Trinity Test, Szilárd and 69 other scientists of the Manhattan Project wrote a petition to President Truman, in which they insisted that the U.S. should use the bomb only to show Japan the power the U.S. now possessed rather than dropping it on a city. The letter was never given to Truman, held off by authorities in charge of the bomb project. The decision to use the weapon on a Japanese city, with a large civilian population, without specific warning, had already been made.

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