The Battle of Passchendaele is synonymous with the horrors and human attrition of the First World War.
Officially known as the Third Battle of Ypres, Passchendaele was fought between July and November 1917 in the West Flanders region of northern Belgium.
Half-a-million soldiers from the Allied and German sides were killed.
On Sunday, foreign dignitaries and descendants of those who died there gathered at the Menin Gate in Ypres to pay their respects.
British prime minister Theresa May, Prince William and his wife Kate Middleton joined the King of Belgium in honouring the dead from the battle.
Prince William reflected on the sacrifices made during the battle and said Britain and Belgium stand together in remembrance.
"The battlefields of the Salient came to define the war for many British and Commonwealth soldiers. The defence of the city at such great cost meant that it became hallowed ground. Winston Churchill said of Ypres, 'A more sacred place for the British race does not exist in all the world.' It was from here, along the Menin Road, that so many marched towards the front line."
Passchendaele became infamous not only because of the scale of the casualties but also because of the mud.
Heavy rains and dysfunctional drainage systems turned the soil into a quagmire, immobilising tanks and drowning men and horses.
The Allied offensive, fought by British and Commonwealth forces, ended up barely moving the front line against the Germans.
Yet, 38,000 Australians, nearly 16,000 Canadians and over 5,000 New Zealanders fell there, either killed, wounded or missing.
During the Gallipoli campaign, by comparison, just over 8,000 Australians died.
Belgium's King Philippe says it is hard to fathom the immense sacrifice of those who fought.
"Every time we stand here under the Menin Gate, we feel overwhelmed by the immensity of the sacrifice of the men whose names surround us. And when a fresh breeze whispers through the arches, it touches something inside all of us. It is as if the fallen were telling us, 'We did this for you.' Indeed, they came to our country from near and far to defend our freedom alongside our own soldiers. Ever since, we have expressed our gratitude to these heroes, and a hundred years have passed without this being diminished."
Thousands of poppies were dropped from the roof of the Menin Gate, representing the names of soldiers that are engraved at the site.
Virginia Crompton is from Passchendaele at Home, an organisation trying to ensure soldiers who died after the battle are properly remembered in Britain.
Ms Crompton has told the B-B-C, although it has now been 100 years since the fighting, it is important to continue to honour those who died
"People were killed outright or wounded terribly. And it's a terrible experience.The scale of the battle is awful. And a hundred years later, we know there's no-one who took part in the battle alive today. If we don't come together to remember in a very special and meaningful way, then those experiences will be lost, and it's very important that we think deeply about that."
A Passchendaele veteran, Richard Tobin, gave this interview back in 1964, offering some insight into the horrific conditions.
"We called it the slaughterhouse. And even the most seasoned veteran felt he'd be lucky if he got there and came back. If you're wounded and you slipped off the duckboards, you just sank into the mud. Not only that, but, deep into every pool, you'd fall into decomposed bodies of humans and humans."
Poet Siegfried Sassoon once wrote, "I died in hell. They called it Passchendaele."
Commemorations included a special service at Tyne Cot cemetery, where thousands are buried.
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