In July 1915 the poet Edward Thomas enlisted as a soldier with the Artists’ Rifles, even though, at the age of 37, he had no obligation to do so. When his friend Eleanor Farjeon asked why, he scooped up a handful of earth and replied: ‘Literally, for this.’ John Lewis-Stempel’s new book is persuasive that Thomas and his contemporaries’ love of the natural world informed both their motivation to fight and their conduct during the first world war.