Russell spent the 1950s and 1960s engaged in political causes primarily related to nuclear disarmament and opposing the Vietnam War. The 1955 Russell–Einstein Manifesto was a document calling for nuclear disarmament and was signed by eleven of the most prominent nuclear physicists and intellectuals of the time. In 1966–1967, Russell worked with Jean-Paul Sartre and many other intellectual figures to form the Russell Vietnam War Crimes Tribunal to investigate the conduct of the United States in Vietnam. He wrote a great many letters to world leaders during this period.