Albert Einstein played a key role in convincing President Franklin D. Roosevelt to launch the Manhattan Project and develop the world's first atomic bomb.
But the renowned theoretical physicist never took part in the secret project run by physicist J. Robert Oppenheimer; US officials were worried his left-leaning political views posed a security threat.
Einstein sent a letter to Roosevelt in August 1939 warning that the Nazis could develop an atomic bomb and recommended "quick action on the part of the Admininstration" — namely, launching its own nuclear program.
The letter cited the Hungarian physicist Leo Szilard's work, and Szilard helped draft the letter, which Einstein signed.
However, the US Army Intelligence office had concerns about Einstein's political ideology and in July 1940 denied him the security clearance to work on the project, according to the American Museum of Natural History.
Intelligence officials also barred the scientists who were part of the program, which was organized by Oppenheimer, from consulting with Einstein.
The Manhattan Project was officially created in August 1942, months after the US entered the war. The years-long program developed the world's first nuclear weapons, which were dropped on Japanese cities Hiroshima and Nagasaki in 1945.
"Woe is me," Einstein said after learning of the attack, according to AMNH.
He later expressed remorse for recommending that the US start its own nuclear program, telling Newsweek, "Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing."
Szilard and his fellow Hungarian-born physicist Eugene Wigner both expressed their agreement with Einstein's statements, according to The New York Times.
The Manhattan Project is the center of a new biopic from director Christopher Nolan. "Oppenheimer," which chronicles the physicist's work developing the nukes and stars Cillian Murphy, releases this week.
