Leó Szilárd
America's rise as a nuclear power resulted directly from the Manhattan Project, codename for a project during World War II to develop the first atomic bombs for wartime use. The project's roots began in 1939 when, at the urging of Leó Szilárd, Albert Einstein signed the Einstein–Szilárd letter to US president Franklin D. Roosevelt expressing his concerns that Nazi Germany may be trying to develop nuclear weapons.