Twice in five years, the threat of a Napoleonic invasion caused real fear in Britain. In 1798, the country was racked by internal divisions, a fiscal crisis, and widespread social unrest--factors that the French invaders hoped would transform an act of aggression into a welcome war of liberation. The invasion never materialized, but in 1803, the renewal of the invasion threat led to an unprecedented mobilization of the British population and an outpouring...