The Battle of Passchendaele was a tactical victory for Allied troops during World War I. Those troops gained the high ground and regained a Belgian town but only after very high casualties, on both sides.

Members of the British Expeditionary Force (BEF) had been in the Ypres area since the beginning of the war, having fought in the Battle of the Frontiers and many subsequent struggles as both sides struggled for supremacy in an increasingly trench-based way of fighting. The 1917 Battle of Passchendaele is also known as the Third Battle of Ypres. The first two battles of Ypres occurred in 1914 and 1915; the latter was the first use of poison gas in the war. Throughout all of that, Allied forces held onto that part of Belgium, known as the Ypres salient.

U.K. commanders had wanted to initiate an attack in Flanders the year before but had agreed to hit the German line further south, on the Somme. In 1917, BEF troops had had some success in the Battle of Arras, capturing high ground along Vimy Ridge. It was then to Passchendaele that Allied commanders looked again. Prime targets were German railroads, faithfully supplying German troops, and German submarine bases, powering the U-boats that still menaced Allied shipping.

The French and BEF troops launched their offensive on July 18. During the next 10 days of artillery bombardment from 3,000 guns, they rained down more than 4 million shells on the German defenders. The first infantry assault came early in the morning on July 31 and achieved very little. Then began some of the heaviest rains the Flanders had seen in three decades. Soldiers were soaked. Guns were drenched. Most importantly for any kind of assault, the land itself was flooded. One consequence of the massive Allied bombardment was the weakening and then destruction of drainage systems for the reclaimed marshland in the immediate area. What remained was a massive area of mud.

The Allied assaults continued, even as did the rains. Tanks got stuck in the mud. Soldiers found it difficult to walk in the muck, which also got into their guns, clogging delivery. Horses struggled to find purchase; the muds claimed the lives of many horses and men. Not made of dirt, of course, were the pillboxes in which armed German defenders waited; those pillboxes were made of concrete and proved more than adequate to the task afforded them.

As the days and weeks piled up, Allied forces continued to press for any kind of advantage. The ground dried in late August. A fierce new assault commenced on September 20 resulted in the seizing of a ridge to the east of Ypres. Subsequent attacks were not at all successful. The German defensive position was still strong, infused with reinforcements from the east; as well, German troops fired mustard gas as well as bullets.

The Allied assaults kept coming, despite the very low rate of success. In the end, their strategy changed, employing a "bite and hold" variant which, rather than trying to force the entire German line back, targeted small parts of the line with one wave of troops after another, taking small amounts of territory and then stopping to reinforce; this strategy allowed artillery to be wheeled into a new position to defend, providing cover for the slowly advancing infantry. By this point in the war, as well, Allied air cover was providing better reconnaissance and attack support.

At last, Allied troops reached Passchendaele, capturing what was left of the village on November 6. German losses were about 250,000 killed or injured. Allied losses were much higher–more than 300,000. The net territorial gain was about five miles.