The First World War began disastrously for the English when the Germans routed them at Mons, Belgium, on 23 and 24 August 1914. Less than a month later, the Anglo-Welsh writer Arthur Machen fictionalized this encounter in a newspaper story, claiming that the English were saved by the appearance of angels, but his newspaper fiction became accepted as fact. The believers - and they included figures as notable as G. K. Chesterton, Arthur Conan Doyle, and C. S. Lewis - wrote pamphlets, testimonies, poems, parodies; they even performed music and created motion pictures attesting to the existence...