The Decision to Begin
In August 1939, physicist Leo Szilard drafted a letter warning President Franklin Roosevelt that Germany might be developing atomic weapons. The letter was signed by Albert Einstein, whose name carried sufficient weight to ensure it was read seriously.
Roosevelt established an advisory committee. Progress was initially slow. The full commitment of American resources came after Pearl Harbor in December 1941 brought the United States into the war. In 1942, the Manhattan Project was formally established under US Army control with J. Robert Oppenheimer as scientific director and General Leslie Groves as military commander.
The decision was made under conditions of genuine uncertainty. Germany's nuclear programme was believed to be advanced. It turned out not to be — but the decision-makers of 1942 could not know that.
The Scale of the Project
The Manhattan Project eventually employed over 130,000 people across multiple sites including Los Alamos in New Mexico, Oak Ridge in Tennessee and Hanford in Washington state. Its total cost in wartime dollars was approximately two billion dollars. Most participants did not know what they were building.
What the Project Required
Building an atomic bomb required solving problems that had never been solved before, using materials that barely existed in usable quantities, on a timeline set by the pace of a global war. Two separate approaches to producing fissile material were pursued simultaneously — enriched uranium and plutonium — because neither had been proven to work at scale.
The industrial infrastructure was extraordinary. Oak Ridge processed uranium on an industrial scale. Hanford produced plutonium. Los Alamos designed the weapons. The secrecy required meant that most workers understood only their specific task and nothing more.
Oppenheimer's challenge at Los Alamos was to assemble the world's leading physicists — many of them refugees from Nazi Europe — and direct them toward a single practical goal under wartime conditions. By most accounts, he did this remarkably effectively.
The Trinity Test
The first nuclear device was tested at Trinity, New Mexico, on 16 July 1945. The yield was approximately 21 kilotons of TNT — larger than many physicists had predicted. Oppenheimer later recalled that the test brought to mind a line from the Bhagavad Gita: "Now I am become Death, the destroyer of worlds." Germany had surrendered two months earlier. The war in Europe was over.
Einstein letter to Roosevelt
Szilard and Einstein warn Roosevelt of German nuclear weapons potential. Advisory committee established.
Pearl Harbor accelerates commitment
US enters the war. Full commitment to atomic weapons programme follows.
Manhattan Project formally established
Oppenheimer appointed scientific director. Groves appointed military commander. 130,000 eventually employed.
Trinity test
First nuclear device successfully detonated at Trinity, New Mexico. Germany has already surrendered.
Hiroshima and Nagasaki
Atomic bombs dropped on Japan. Japan surrenders. The Second World War ends.
The Decision to Use
With Germany defeated, the question became whether and how to use the weapons against Japan. The stated justification was that the bombs would end the war without the casualties projected for an invasion of the Japanese home islands — estimates ranged from hundreds of thousands to over a million.
Critics have argued that Japan was already close to surrender and that the bombings were unnecessary, or that they were partly motivated by a desire to end the war before Soviet involvement in the Pacific deepened. The historical debate is genuine and unresolved. Scholars including Tsuyoshi Hasegawa have argued that the Soviet declaration of war against Japan on 8 August — between the two bombings — may have been more decisive in Japan's surrender calculation than the bombs themselves.
An estimated 70,000 to 80,000 people died immediately at Hiroshima, with similar numbers at Nagasaki. Many more died from injuries and radiation in the following months and years.
The World the Project Created
Japan surrendered on 15 August 1945. The Second World War was over. The Manhattan Project had achieved its immediate objective.
It had also created something that could not be uncreated. The Soviet Union tested its first atomic bomb in 1949. The nuclear arms race that followed shaped international relations for the remainder of the twentieth century. The project that began as a response to a feared German weapon produced a permanent transformation in the nature of military power and international relations.
The scientists and decision-makers of 1942 were responding to a real threat as best they understood it. The world they created was one none of them had fully anticipated.
Frequently Asked Questions
What was the Manhattan Project?
The Manhattan Project was the Allied scientific and military programme that developed the first nuclear weapons during World War Two. Running from 1942 to 1945, it employed over 130,000 people across multiple sites and produced the bombs dropped on Hiroshima and Nagasaki in August 1945.
Why did the United States develop the atomic bomb?
The primary stated motivation was fear that Nazi Germany was developing nuclear weapons first. Scientists including Leo Szilard and Albert Einstein warned Roosevelt of this possibility in 1939. Germany surrendered before the bombs were ready, raising questions about whether the original rationale still applied to their use against Japan.
Was the decision to use the atomic bomb on Japan justified?
This remains one of the most debated questions in modern history. Proponents argue the bombs ended the war without an invasion that would have caused greater casualties on all sides. Critics argue Japan was close to surrender and the bombings were unnecessary, or were partly motivated by Cold War strategic considerations. Historians continue to disagree, and the question has no settled answer.
How many people died at Hiroshima and Nagasaki?
Estimates vary and remain debated. An estimated 70,000 to 80,000 people died immediately at Hiroshima, with similar numbers at Nagasaki. Total deaths including those from radiation and injuries in subsequent months are estimated at 130,000 to 226,000 across both cities, though precise figures are difficult to establish.
Kai Bird and Martin Sherwin's biography of Oppenheimer argues that the decision to use the bombs was less inevitable than the postwar narrative suggests — and that the political and military pressures shaping the decision were more complex than a simple calculation of lives saved. This is one of those topics where the more you read, the more complicated it gets.
A Note From The Editor
What strikes me about the Manhattan Project is not the physics or even the destruction. It is the gap between what the decision-makers intended and what they created. The project began as a defensive response to a threat that turned out not to exist at the level feared. It ended by permanently altering the logic of international conflict. The scientists who built the bomb understood the physics. Nobody fully understood what the world would look like afterward — including them.