Albert Einstein was a German-born theoretical physicist. He wasn’t allowed to work on the Manhattan Project because of concerns about him being a security risk due to his birthplace and political ideology, but he did play a role in the steps leading to the project.
He was living in the United States in 1939 at the start of WWII, and he heard about the work taking place in Germany to build an atomic bomb. He sent a letter to President Franklin Delano Roosevelt dated August 2, 1939, warning him of the Germans’ efforts and encouraging him to consider joining the race to build an atomic bomb. President Roosevelt responded by thanking him for the suggestion, but more than two years passed before the U.S. entered WWII in 1941. President Roosevelt then remembered Einstein’s advice, and decided it would be wise to begin the work to build an atomic bomb. He launched the secretive Manhattan Project, with several scientists that were colleagues of Einstein’s.
Einstein did not create nuclear weapons, but his famous equation E=mc2 explains the energy released in an atomic bomb. However, he was often incorrectly associated with the creation of the atomic bomb. He was known to remind people, “I do not consider myself the father of the release of atomic energy. My part in it was quite indirect.” In an interview with Newsweek magazine, he said, “Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing,” meaning that he came to regret writing his letter to the President.