The Duke of Cambridge has led official commemorations marking the 100th anniversary of the Battle of Passchendaele.
William spoke at the Menin Gate monument in Ypres, Belgium, as the daily Last Post was played ahead of Monday's centenary of the start of the bloody First World War British assault.
Flanked by the Duchess of Cambridge and Philippe and Mathilde, the King and Queen of the Belgians, he said Britain and Belgium "stand together" to remember those killed during weeks of heavy fighting in the summer and autumn of 1917.
Watched by some 200 descendants of those who fought, he said: "Today, the Menin Gate records almost 54,000 names of the men who did not return home; the missing with no known grave.
"Members of our families; our regiments; our nations; all sacrificed everything for the lives we live today."
He added: "During the First World War Britain and Belgium stood shoulder to shoulder. One hundred years on, we still stand together, gathering as so many do every night, in remembrance of that sacrifice."
Sunday's poignant Last Post was the 30,752nd time it has been played since 1928.
The towering Menin Gate in the Belgian town is covered with the names of 54,391 British dead who have no known grave, according to the Commonwealth War Graves Commission.
In just over three months of conflict there were more than half a million casualties - 325,000 Allied soldiers and 260,000 to 400,000 Germans - in the Belgian battlefields.
It was fought between July 31 and November 10 1917 in battlefields that were summed up in poet Siegfried Sassoon's line "I died in hell, they called it Passchendaele".
Those who fought there included Harry Patch, the "Last Tommy" who died aged 111 in 2009.
Catherine, dressed in a cream Alexander McQueen dress, stood between William, who wore a blue suit with medals, and a more sombrely dressed Prime Minister Theresa May.
They watched as thousands of paper poppy petals, one for every name on the Menin Gate, fluttered to earth from the roof above the gathered crowd.
They included some 200 descendants of those who fought at Passchendaele.
Four thousand people were chosen by a ballot to attend events in Ypres on Sunday and the larger event centred on nearby Tyne Cot military cemetery on Monday.
The royals and Mrs May later attended a brief reception at Ypres Cloth Hall, meeting relatives of those killed in the battle.
The duchess spoke to Scottish singer-songwriter Davy Holt, who found out 10 years ago that his great uncle John Kimm is remembered on the Menin Gate.
Mr Holt, from Inverness, who wrote a song about his relative called Hero Of Your Time, said: "She asked me what his story had done for me.
"I couldn't really understand why my family has spent 90 years not talking about it.
"I decided I would talk about it, and do so by writing a song."
The evening rounded off with performances in Ypres's Grote Markt square, which features Dame Helen Mirren and journalist Ian Hislop, who introduced a sketch from his First World War play The Wipers Times.
Testimony from Allied and German soldiers was also projected onto the side of the imposing Cloth Hall.