Einstein's theory laid a theoretical foundation for the development of nuclear energy. In order to help fight against the Nazis, he wrote to President franklin roosevelt with the help of Leo Szilard and others, which directly contributed to the start of the Manhattan Project. After World War II, he actively advocated peace and opposed the use of nuclear weapons, and signed the Russell-Einstein Declaration. Einstein initiated a new era of modern science and technology and was recognized as the greatest physicist after Galileo and Newton.