The Biggest Error in Modern History

The First World War has all but passed out of living memory. Yet its hold on the imagination is as powerful as ever. Why is this? The answer may lie in the Battle of the Somme, the engagement that more than any other has come to symbolise the futility and slaughter of the Western Front. On its 90th anniversary, John Lichfield reports from the tiny strip of northern France where a quarter of a million men lost their lives in just four months