On a summer morning in Sarajevo almost a hundred years ago, a teenager took a pistol out of his pocket and fired not just the opening rounds of the First World War but the starting gun for modern history. By killing Archduke Franz Ferdinand, the heir of the Austro-Hungarian Empire, Gavrilo Princip, started a cycle of events that would leave 15 million dead from fighting between 1914 and 1918 and proved fatal for empires and a way of ruling that had...