Every late autumn, the artificial red blossoms appear without fail, fashioned from fabric or felt, enamel or plastic. They are affixed to the lapels of newscasters, the team uniforms of footballers, the rumpled raincoats of Tube passengers. Even the queen dons a scarlet poppy brooch. The poppy pins are meant to commemorate military sacrifice in all the country’s wars. But they are most closely associated with a conflict whose wounds still linger more than a century after its end — World War I, when modern battlefield weapons met antiquated customs of warfare, to devastating effect. Some three-quarters of a million British soldiers lost their lives in what was then called the Great War. Including combatants drawn from across the British Empire, the fatality figure rises to nearly 1 million. “The First World War persists in our public mind-set and remains this kind of trauma,” said Laura Clouting, a senior curator at the Imperial War Museum in London who specializes in World War I. “The sheer numbers of men and women who died — for Britain, these losses were unprecedented and never matched since.” Working-class recruits bore the brunt of the casualties in terms of total numbers. ...