Next month will see the 80th anniversary of the Allied invasion of Nazi-occupied France, when about 156,000 troops crossed the Channel to fight their way on to five Normandy beaches. No one who took part in that day, 6 June 1944, the largest seaborne assault in history, would ever forget the experience. Indeed, many were haunted by memories of it for the rest of their lives. Yet no matter how momentous an event might be – in this case it amounted to nothing less than the securing of western Europe’s liberty – a kind of societal amnesia inevitably deepens with...