Today marks 100 years since the start of the Battle of Passchendaele.
The Prince of Wales, the Duke and Duchess of Cambridge and the Prime Minister were among senior royals and politicians attending a ceremony to mark the occasion.
The fighting lasted more than 100 days and left more than half a million men dead or injured on both sides.
It was one of the bloodiest offensives of the First World War.
The commemoration took place at Tyne Cot cemetery near Ypres in Belgium.
This is the largest Commonwealth burial ground with almost 12000 servicemen buried there, but only 8,373 are identified.
Charles spoke of the "courage and bravery" of British soldiers killed at Passchendaele:
"We remember it not only for the rain that fell, the mud that weighed down the living and swallowed the dead, but also for the courage and bravery of the men who fought here."
General Sir Nick Carter, Chief of the General Staff says the Battle of Passchendaele helps the British public reflect on modern day conflict:
Last night, it was Prince William who spoke as the Last Post was played at Menin Gate.
Buglers from the local fire brigade sound the Last Post there every evening at 8pm.
This ceremony has been uninterrupted since 2nd July 1928, except for the during the German occupation in the Second World War, when the daily ceremony was conducted at Brookwood Military Cemetery, in Surrey.
That made Sunday's Last Post the 30,752nd time it's been played.
Watched by some 200 descendants of those who fought, he said: "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."
This service was then followed by a show which saw Cloth Hall illuminated with projections and lights, together with a special live performance from artists including Dame Helen Mirren, Alfie Boe, War Horse, and the cast of The Wipers Time.
Adm Sir Philip Jones, First Sea Lord says he pleased to be commemorating the sacrifices those involved in the Battle of Passchendaele made:
Air Chief Marshal Sir Stephen Hillier, Chief of the Air Staff says the Battle of Passchendaele had a key role to play in the formation of The Royal Air Force:
Alongside Royals and politicians, relatives of soldiers involved in the Battle of Passchendaele are also in Belgium for the commemorations.
In 1917 Nelly Ayres received a telegram from the British Army telling her that her husband Arthur has been made "non-effective by death".
A Royal Engineer, Sapper Ayres died in a military hospital in France three weeks after being injured at Passchendaele, caring for injured comrades.
100 years later and his granddaughter Sue Patterson is in Belgium remembering his sacrifice.
A letter from her grandfathers Commanding Officer had revealed that Sapper Ayres, after being wounded himself, started helping bandage other injured comrades where he was struck again by a second artillery blast.
Sue Patterson is among 4,000 descendants of those who fought attending the memorial services.
Some of those who have come have relatives buried in visitable graves, while thousands whose bodies were never recovered are remembered in names etched into plaques on the walls of Tyne Cot Cemetery.
The descendants will later pay tribute to their relatives and the thousands of others who fell at Passchendaele.