The Duke and Duchess of Cambridge will join Prime Minister Theresa May in Belgium later to mark the centenary of the Battle of Passchendaele – one of the bloodiest of World War One.
Half a million Allied and German soldiers were killed, wounded or went missing in three months of fighting.
Many drowned in the thick mud, caused by weeks of relentless rain.
A service will be held in Ypres, where the King of Belgium and Prince William will lay wreaths at the Menin Gate.
They will hear the Last Post, which has been played at the gate by a bugler almost every evening since 1928.
Dignitaries and the descendants of those who died will gather in Ypres’s Market Square for an event to tell the story of the battle.
Performances and music – which will include the National Youth Choir of Scotland, Dame Helen Mirren and a specially written extract from War Horse, narrated by Michael Morpurgo – will be set to a backdrop of light projections on to the historic Cloth Hall.
Officially known as the Third Battle of Ypres, Passchendaele was fought between 31 July and 6 November 1917 in the West Flanders region of northern Belgium.
About 275,000 Allied troops and 220,000 Germans died.
British troops sought to push back the Germans from a ridge, near the Belgian town of Ypres during the three-and-a half month campaign.
Thick mud clogged up rifles and immobilised tanks, and ultimately, the fighting is believed to have won the allied forces only small gains.
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On Monday, the anniversary of the start of the battle, commemorations will continue with a special service held at Tyne Cot cemetery, where thousands are buried and commemorated.
World War One Remembered: Passchendaele For The Fallen is on BBC Two on Sunday 30 July at 7pm.