Although the entire Third Battle of Ypres is normally known as Passchendaele, that name officially belongs to two battles late in the campaign. After a period of failure at the start of the battle, late September and early October had seen some British success. General Herbert Plumer had abandoned the idea of the breakthrough battle in favour of a series of “bite and hold” battles, each designed to take a chuck out of the German lines. In dry weather his Second Army had won a series of victories on the Menin Road Ridge (20-25 September 1917), at Polygon Wood (26-27 September) and at Broodseinde (4 October). The German lines had been pushed back to the edge of the Passchendaele ridge.
The dry period came to an end after the first week of October. The British attack, at Poelcapelle on 9 October, came after 25mm of raid fell over two days. That may not seem a vast amount of rain, but it did represent close to half of the expected average rainfall for October. Worse, the flat ground around Ypres relied on its field drainage system to stay dry. Three years of constant bombardment of the area around Ypres had destroyed that system, while also churning up the soil. The attack at Poelcapelle made some limited progress, but began bogged down in the mud.
The rain continued over the next few days. Another 14mm of raid fell between 10 and 12 October. Any further British advance would have to be made over (or through) fields of mud.
The successes of late September had been won with the aid of a well planned artillery bombardment. A similar bombardment was planned for the attack on Passchendaele, but the mud dramatically reduced its impact. Shells buried themselves in the mud, and either failed to explode at all or had the force of the explosion absorbed by the mud.
Haig did have some fresh troops. The 3rd Australian Division and the New Zealand Division had seen relatively little combat, and were allocated the task of leading the assault on Passchendaele.
The attack on 12 October was a total failure. Part of the 3rd Australian Division came under German artillery fire before the attack even started, causing confusion. Forward patrols reached Passchendaele village, but were not strong enough to hold the village and were soon forced to retreat to their starting point. The 10th Australian Brigade was stopped by machine gun fire from its flank. The New Zealand Division ran into unbroken German wire and suffered heavy losses (nearly 3,000 men) attempting to pass through a single gap in the wire. At the end of the day, all of the attacking units had been forced to pull back almost to their original position.