The Duke and Duchess of Cambridge will join Prime Minister Theresa May in Belgium later to mark 100 years since the Battle of Passchendaele during 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 rain.
A service will be held in Ypres, where the King of Belgium and Prince William will lay wreaths at the Menin Gate.
Officially known as the Third Battle of Ypres, Passchendaele was fought between 31 July and 6 November 1917 in northern Belgium.
British troops hoped 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 stopped tanks, and ultimately, the fighting is believed to have won the allied forces only a small amount of ground.
On Monday, the anniversary of the start of the battle, events will continue with a special service held at Tyne Cot cemetery, where thousands are buried and commemorated.