TURNING POINTS
How FDR Set the Manhattan Project in Motion—In Secret
A warning from European physicists became a wartime program of hidden appropriations, Army construction, and decisions held inside a remarkably small circle.
A warning carried to the White House in 1939 began as a $6,000 uranium study. Within three years, Roosevelt had approved a secret, Army-backed atomic-weapons program.
On October 11, 1939, Alexander Sachs brought President Franklin D. Roosevelt a letter signed by Albert Einstein and drafted by Leo Szilard. It warned that uranium research could lead to an extraordinarily powerful weapon and raised the prospect of German work in the same field. [2]
Roosevelt did not immediately create the vast atomic-bomb enterprise Americans would later know as the Manhattan Project. His first response was the Advisory Committee on Uranium, which received $6,000 for graphite and uranium experiments. [2]
That small appropriation marked the start of a chain of decisions that shifted atomic research from an open scientific question into a secret military-industrial undertaking.
A Warning, Not a Blueprint
The Einstein-Szilard letter gave Roosevelt a reason to investigate, not a finished plan for a weapon. Its warning carried unusual weight because Einstein had signed it, but Szilard had drafted the message and pressed for action. [2]
For more than a year, the American effort remained limited. The central question was still whether a bomb was technically possible and, if so, whether it could be built in time to matter during the war.
Einstein’s part in the story was important but narrow: he signed the 1939 warning letter and did not work on the Manhattan Project itself. [2]
Britain Raises the Stakes
A British assessment changed the calculation. In July 1941, the MAUD Committee concluded that a critical mass of uranium-235 could produce an operational weapon within the likely span of World War II. [1]
On October 9, 1941, Roosevelt met privately with Vannevar Bush and Vice President Henry Wallace. He authorized accelerated inquiry into bomb development and its likely costs. [1]
The meeting came before the Japanese attack on Pearl Harbor. Roosevelt was already allowing planners to examine what it would take to turn the scientific possibility into a production program.
“Keep This in Your Own Safe”
Roosevelt made his approval unmistakable on January 19, 1942. Replying to Bush, he wrote: “V. B. OK — returned — I think you had best keep this in your own safe.” [1]
The note was brief, but its instruction reflected the project’s operating principle. Atomic work would be tightly held within a small executive circle, with information restricted in the name of security. [3]
Bush soon made clear that research alone would not settle the matter. Building a weapon could require full-scale production plants, while American officials feared they might be racing German efforts. [5]
The money would remain hidden as well. By 1945, the Manhattan Project had cost roughly $2 billion, concealed within War Department appropriations through confidential arrangements with key congressional leaders. [6]
The Army Takes Over Construction
The project required an organization capable of building and managing enormous production facilities. On June 17, 1942, Bush recommended that the Army Corps of Engineers take responsibility for plant construction. The Army established the Manhattan Engineer District on August 13. [3]
The move turned an urgent research program into a major military construction effort. It also created the administrative framework for the secret enterprise that would become known as the Manhattan Project.
On December 2, 1942, Enrico Fermi’s team achieved the first controlled, self-sustaining nuclear chain reaction at Chicago Pile-1 beneath Stagg Field at the University of Chicago. [4]
That result did not produce a bomb. It did demonstrate that a controlled chain reaction could be sustained—an essential scientific step as the United States committed resources to the larger project.
What Roosevelt’s Decisions Changed
Between 1939 and 1942, Roosevelt moved the United States from a modest uranium investigation to an accelerated, closely guarded atomic-weapons program. The path ran through a refugee scientist’s warning, British technical findings, private meetings, a handwritten approval, and an Army engineering district. [1][2][3]
The secrecy held so tightly that Vice President Harry S. Truman had not been told about the Manhattan Project before Roosevelt died on April 12, 1945. Truman learned of it only after becoming president. [6]
That was the consequence of the system Roosevelt and his advisers had built: a project large enough to absorb billions of dollars, yet compartmentalized enough that even the incoming president had been outside it.
TIMELINE
Key moments
Sources
- A Tentative Decision to Build the Bomb - Manhattan Project: An Interactive History — U.S. Department of Energy Office of History and Heritage Resources
- Einstein-Szilard Letter — Atomic Heritage Foundation / National Museum of Nuclear Science & History
- Vannevar Bush to President Roosevelt, 9 March 1942, with memo from Roosevelt attached, 11 March 1942 — National Security Archive, George Washington University
- Manhattan Project Notebook (1942) — U.S. National Archives and Records Administration
- Bush-FDR Letter - March 9, 1942 — Atomic Heritage Foundation / National Museum of Nuclear Science & History
- Manhattan Project Historical Resources — U.S. Department of Energy
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