The nuclear endgame to World War II triggered a widespread sense that mankind had reached the crossroads. ‘It must be made impossible for war to begin, or else mankind perishes,’ warned The Times of London. Bertrand Russell wrote: ‘The human race has to choose between utter disaster and unexampled well-being – no middle course is any longer possible.’ The title of the manifesto of the American atomic scientists provided one of the commonplace phrases of the early nuclear age: One World or None? The notion that civilization was in a dangerous phase of ‘technological adolescence’ became widespread: it was a case of grow up or blow up. Paradoxically, fear of nuclear war generated visions of progress as thinkers and activists sought to construct alternative scenarios for survival: the world government movement of the later 1940s; the ‘atoms for peace’ utopia of unlimited energy and atomic-powered transport; the drive for space travel, which would ensure human survival beyond the Earth; and contact with advanced extraterrestrial civilizations, which would by definition have overcome their own atomic crisis and be anxious to transmit the formula for survival to homo sapiens. Progress beyond the atomic crossroads would be rapid and unlimited, and the favourite strategy was expansion into space. The progressive technological optimism of the 1950s and 1960s, however, was always the anxious twin of an alternative scenario: that of nuclear extinction. The assumption that modern civilization stood at an atomic crossroads pervaded the early nuclear age, with interesting effects on scientific and cultural thought.