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Why did the U.S. government keep Albert Einstein out of the Manhattan Project?

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Portrait of Albert Einstein in a suit and tie, looking slightly to the side with a neutral expression. — Albert Einstein Albert Einstein developed the special and general theory of relativity, which recast gravity as a bending in space-time.

Although Albert Einstein’s work and his famous equation, E = mc², were foundational to the development of the atomic bomb, he was not invited to participate in the Manhattan Project.

The U.S. government excluded him due to concerns regarding his political background. Declassified Federal Bureau of Investigation (FBI) files indicate that officials feared his lifelong associations with pacifist and socialist organizations. FBI director J. Edgar Hoover even attempted to prevent Einstein from entering the United States under the Alien Exclusion Act, though this effort was ultimately overruled by the U.S. State Department.12

Despite this exclusion, Einstein contributed to the broader war effort by helping the U.S. Navy evaluate designs for future weapons systems and by raising funds through the auction of his personal manuscripts.1