YPRES, Belgium (AP) — Dismembered soldiers sucked into cesspools of mud. Shattered tree trunks and the waft of poison gas hovering over the wounded who were awaiting their fates on the scarred soil of Flanders Fields.

The Third Battle of Ypres, fought in western Belgium a century ago, was as bad as World War I would get. Half a million soldiers were estimated to have been killed or wounded during the 100-day battle and one name keeps coming back: Passchendaele, now as grim a symbol as any field of war ever remembered.

Monday marks the centennial of the start of the Allied offensive, which ende...