The Third Battle of Ypres - also known as Passchendaele - was fought between July and November 1917.
It has shaped perceptions of the First World War on the Western Front, with both sides suffering heavy casualties and enduring appalling conditions. The name Passchendaele has become synonymous with mud, blood and futility.
General Sir Douglas Haig had planned a major offensive to break out of the Ypres salient, which the Allies had occupied since 1914.
Haig's vision was for a war-winning breakthrough. He planned to capture the high ground around Ypres, as well as a key rail junction to the east, and then advance on the German-occupied ports of the Belgian coast - critical to the U-Boat campaign.
The battle failed to achieve Haig's objectives. It lasted over 100 days. In that time, the Allies advanced about 5 miles for the loss of over 250,000 soldiers killed, wounded or missing.
The Battle began on 31 July 1917
After a two-week artillery bombardment, the Third Battle of Ypres began on 31 July.
On the first day, British and French forces made significant gains in some sectors. As the day wore on, it began to rain heavily.
The rain made it difficult for the artillery to support the advance. The German defensive system was designed to wear attacking forces down before pushing them back with powerful counterattacks.
Having advanced deep into this system, and lacking effective artillery support, the Allies were vulnerable to these counterattacks and lost much of the territory they had gained.
Rain and mud defined the battle
The rain began on 31 July - the first day of the battle. The battlefield had been churned up by the Allied artillery bombardment, destroying the ditches that acted as a drainage system.
The rain continued for three weeks. Soldiers struggled through heavy, sticky mud that reached up to their waists. Men, horses, tanks and other vehicles literally drowned in the mud.
It was almost impossible to manoeuvre artillery into new positions, and aerial reconnaissance was grounded by the poor visibility.
Stages of the battle were successful
The Third Battle of Ypres is perceived as muddy, bloody and futile. The early and latter stages of the battle largely conform to this image but the middle stages were very successful.
In September, General Herbert Plumer replaced General Hubert Gough as commander. He introduced a new approach.
Instead of attacking along a wide front and setting ambitious objectives deep inside the German defensive system, Plumer favoured a 'bite and hold' strategy.
Better weather and better tactics
He set limited objectives, making sure the attack was always within range of artillery support. Once they reached their objective, troops dug in to prepare for counterattacks.
Plumer's new approach was also helped by improved weather conditions in September. Better weather meant more artillery and aerial reconnaissance to support the attack.
Although they continued to suffer heavy casualties, the Allies made significant progress at the Menin Road, Polygon Wood and Broodseinde.
Passchendaele was the final objective
The Third Battle of Ypres is often called Passchendaele. It's been referred to by this name since the 1920s. The village of Passchendaele - and Passchendaele Ridge - were objectives in the final stages of the offensive.
The rain returned in October. Despite worsening conditions and exhausted troops, Haig decided to continue the offensive in order to capture Passchendaele Ridge.
The heavy casualties and appalling conditions suffered during the advance on Passchendaele perhaps contributed to the use of the name to refer to the battle as a whole.
Casualties were heavy
The Allies suffered over 250,000 casualties - soldiers killed wounded or missing - during the Third Battle of Ypres. Casualties among German forces were also in the region of 200,000. The Commonwealth War Graves Commission commemorates over 76,000 soldiers who died during the Third Battle of Ypres. More than half have no known grave.
The German Army could not afford the losses it incurred at Ypres. While the Allies would soon be reinforced by the Americans, Germany could not replace that manpower.
