Field Marshal Douglas Haig wanted to break out of the Ypres Salient, capture the Belgian coast, and destroy the German submarine bases threatening to starve Britain into submission. The War Cabinet reluctantly approved the offensive on July 20, 1917, but Prime Minister David Lloyd George opposed it, as did French General Ferdinand Foch. The attack would come after the Battle of Messines in June secured the high ground south of Ypres, where British miners detonated nineteen enormous mines beneath the German lines. The main offensive, originally planned for the end of July, would advance toward Passchendaele Ridge, then push on to cut the railways supplying the German Fourth Army. An amphibious landing on the coast would follow.

The attack began on July 31 with the Battle of Pilckem Ridge. The French First Army and northern British corps advanced successfully, but on the crucial Gheluvelt Plateau to the south, the assault stalled against the main German defensive concentration. Then the rains came. August 1917 was one of the wettest on record, dropping over three inches of rain on ground already shattered by millions of shells. The elaborate drainage system of Flanders had been destroyed by bombardment. Craters filled with liquid mud. Tanks sank to their turrets. Men and horses drowned in the morass. The attacks of August gained little ground at enormous cost, and Haig transferred principal responsibility to General Plumer and the Second Army.

Plumer transformed the offensive. He limited objectives to what artillery could support and troops could hold against German counter-attacks. The bite-and-hold method produced three successive victories in September and early October: the Battle of the Menin Road Ridge on September 20, the Battle of Polygon Wood on September 26, and the Battle of Broodseinde on October 4. Each attack advanced about 1,500 yards on narrow fronts behind devastating artillery barrages. German counter-attack divisions, which had recaptured lost ground throughout August, now arrived to find British troops dug in on tactically advantageous positions. The German official history called October 4 the black day. It seemed the ridge would fall.

The rains returned. The Battle of Poelcappelle on October 9 and the First Battle of Passchendaele on October 12 turned into disasters. New Zealand troops suffered nearly 3,000 casualties on October 12 alone, one of the worst days in that nation's military history. The mud that had dried enough to support the September offensives returned with a vengeance. Artillery could not be moved forward. Ammunition supply broke down. The wounded drowned in shell holes before stretcher bearers could reach them. Yet Haig pressed on, convinced that German morale was cracking and that one more push would secure the ridge for winter.

The Canadian Corps arrived in mid-October to relieve the exhausted Anzacs. Lieutenant General Arthur Currie planned three careful attacks for October 26, October 30, and November 6. The Canadians advanced methodically, consolidating each gain before pushing forward again. On November 6, the 1st and 2nd Canadian Divisions captured what remained of Passchendaele village. A final attack on November 10 secured the remaining high ground north of the village. The campaign was over. The ridge had fallen, but the coast remained in German hands. The strategic objectives that had justified the offensive were never achieved. The controversy over whether the human cost was worth the result began immediately and has never ended.