Officially known as the Third Battle of Ypres, Passchendaele became infamous not only for the scale of casualties, but also for the mud.

Ypres was the principal town within a salient (or bulge) in the British lines and the site of two previous battles: First Ypres (October-November 1914) and Second Ypres (April-May 1915). Haig had long wanted a British offensive in Flanders and, following a warning that the German blockade would soon cripple the British war effort, wanted to reach the Belgian coast to destroy the German submarine bases there. On top of this, the possibility of a Russian withdrawal from the war threatened German redeployment from the Eastern front to increase their reserve strength dramatically.

The British were further encouraged by the success of the attack on Messines Ridge on 7 June 1917. Nineteen huge mines were exploded simultaneously after they had been placed at the end of long tunnels under the German front lines. The capture of the ridge inflated Haig's confidence and preparations began. Yet the flatness of the plain made stealth impossible: as with the Somme, the Germans knew an attack was imminent and the initial bombardment served as final warning. It lasted two weeks, with 4.5 million shells fired from 3,000 guns, but again failed to destroy the heavily fortified German positions.

The infantry attack began on 31 July. Constant shelling had churned the clay soil and smashed the drainage systems. The left wing of the attack achieved its objectives but the right wing failed completely. Within a few days, the heaviest rain for 30 years had turned the soil into a quagmire, producing thick mud that clogged up rifles and immobilised tanks. It eventually became so deep that men and horses drowned in it.

On 16 August the attack was resumed, to little effect. Stalemate reigned for another month until an improvement in the weather prompted another attack on 20 September. The Battle of Menin Road Ridge, along with the Battle of Polygon Wood on 26 September and the Battle of Broodseinde on 4 October, established British possession of the ridge east of Ypres.

Further attacks in October failed to make much progress. The eventual capture of what little remained of Passchendaele village by British and Canadian forces on 6 November finally gave Haig an excuse to call off the offensive and claim success.

However, Passchendaele village lay barely five miles beyond the starting point of his offensive. Having prophesied a decisive success, it had taken over three months, 325,000 Allied and 260,000 German casualties to do little more than make the bump of the Ypres salient somewhat larger. In Haig's defence, the rationale for an offensive was clear and many agreed that the Germans could afford the casualties less than the Allies, who were being reinforced by America's entry into the war. Yet Haig's decision to continue into November remains deeply controversial and the arguments, like the battle, seem destined to go on and on.