An аtоmic bоmb cоuld destroy аn entire city, but а relatively few Hydrogen bombs threatened to destroy entire nations, and early in the 1950s, the threat of radioactive fallout from a war fought with Hydrogen bombs appeared to endanger most forms of complex life (the threat of nuclear famine caused by a global stratospheric smoke layer was yet unknown). Scientists were the first to realize this; in 1955. Lord Bertrand Russell and Albert Einstein, along with some of the most famous scientists of the 20th century, published a Manifesto warning that nuclear war threatened continued human existence.