Answer:
When Einstein learned that the Germans might succeed in solving these problems, he wrote to President Franklin Roosevelt with his concerns.
In August 1939, Einstein wrote to U.S. President Franklin Roosevelt to warn him that the Nazis were working on a new and powerful weapon: an atomic bomb. Fellow physicist Leo Szilard urged Einstein to send the letter and helped him draft it.
In an interview with Newsweek magazine, Einstein said that "had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing."
Explanation: All from American Museum of Natural History