The historian A.J.P. Taylor wrote one of the most powerful openings of any book. “In the state of nature which Hobbes imagined, violence was the only law, and life was ‘nasty brutish and short’,” he wrote in The Struggle for Mastery in Europe, 1848-1918. “Though individuals never lived in this state of nature, the Great Powers of Europe have always done so. Sovereign states have distinguished European civilisation, at any rate since the end of the fifteenth century. Each individual state in...