Field Marshal Douglas Haig aimed to break out of the Ypres salient and push toward the Belgian coast. A preliminary bombardment of more than four million shells wrecked the drainage of the low-lying land, and heavy rain began on the first afternoon. The battlefield became a swamp in which men and horses drowned. When Canadian troops finally took the ruined village of Passchendaele in November, the Allies had advanced about eight kilometers (five miles) at a cost of roughly 250,000 or more casualties.
By mid-1917, the French army had been shaken by mutinies after the failed Nivelle Offensive, and Haig believed a British offensive in Flanders could relieve pressure on it and capture the German submarine bases on the Belgian coast. In June, the British had seized Messines Ridge south of Ypres after exploding huge mines beneath German lines.
The main attack, the Battle of Pilckem Ridge, began at 3:50 a.m. on July 31, 1917. Low cloud meant it was still dark as the infantry advanced. They made gains in the north, but German counterattacks retook ground in the center. That afternoon it began to rain, and August 1917 turned out to be one of the wettest in decades. Shell craters filled with water, and duckboard tracks became the only way to move.
The offensive continued in stages into November. Under General Herbert Plumer, a series of shorter, limited attacks in September gained ground, but the rain returned in October. The final assaults on Passchendaele ridge were made by Canadian troops. The poet Siegfried Sassoon later wrote, "I died in hell / (They called it Passchendaele)."
Key people: Douglas Haig, Hubert Gough, Herbert Plumer