Passchendaele epitomises everything that was most terrible about the Western Front. The photographs never sleep of this four-month battle, fought from July to November 1917, the worst year of the war: blackened tree stumps rising out of a field of mud, corpses of men and horses drowned in shell holes, terrified soldiers huddled in trenches awaiting the whistle. The intervening century, the most violent in human history, has not disarmed these pictures of their power to shock. At the very least they ask us, on the 100th anniversary of the battle, to see and to try to understand what happened here. Yes, we commemorate the event. Yes, we adorn our breasts with poppies. But have we seen? Have we understood? Have we dared to reason why? What happened at Passchendaele was the expression of the 'wearing-down war', the war of pure attrition at its most spectacular and ferocious.