Einstein left Germany for good in 1933, the year Hitler took power, and settled at the Institute for Advanced Study in Princeton, where he spent the rest of his life. He had been a committed pacifist for decades, but the rise of Nazism led him to a harder position: in 1939, alarmed that German scientists might build an atomic weapon using the nuclear fission just discovered by Otto Hahn and Fritz Strassmann, he signed a letter, largely drafted by the physicist Leo Szilard, urging President Franklin Roosevelt to fund research into a bomb before Germany could build one first. The letter helped set in motion what became the Manhattan Project, though Einstein himself, denied a security clearance because of his earlier pacifist and left-leaning associations, took no part in the weapon's design. He spent much of his later life arguing for nuclear disarmament and international control of atomic weapons, work that culminated in the Russell-Einstein Manifesto, signed days before his death in 1955, warning of the risk nuclear war posed to humanity as a whole.
Einstein was equally outspoken on civil rights in his adopted country. He joined the NAACP, corresponded and appeared publicly with the singer and activist Paul Robeson and the scholar W. E. B. Du Bois, and in a 1946 speech at Lincoln University, a historically Black college, called racism America's worst disease. In 1952 the newly founded state of Israel offered him its presidency, a largely ceremonial post, he declined, saying he lacked the aptitude and experience for public office. Across all of it, Einstein treated his scientific authority as a platform for moral argument rather than something to keep separate from public life, an approach that drew both admiration and, especially during the McCarthy era, suspicion from an FBI file that ran to more than 1,400 pages.