in 1933, einstein was visiting the united states but in germany, adolf hitler and the nazis came to power (this is before world war ii). einstein, being of jewish ethnicity, did not return to germany due to hitler’s anti-semitic policies. he lived in the united states and became an american citizen in 1940. on the beginning of world war ii, he sent a letter to president franklin d. roosevelt explaining to him that germany was in the process of making a nuclear weapon; so einstein recommended that the u.s. should also make one. this led to the manhattan project, and the u.s. became the first nation in history to create and use the atomic bomb (not on germany but on japan). einstein and other physicists like richard feynman who worked on the manhattan project later regretted that the bomb was used on japan.