The Queen was there to pay tribute to the tens of thousands who died
The Queen and the Duke of Edinburgh attended a commemoration service marking the 90th anniversary of the battle of Passchendaele.
The service at Tyne Cot cemetery, near Ypres, was also attended by Belgium's Queen Paola and around 4,000 locals.
Almost 11,000 servicemen are buried in the Commonwealth war cemetery.
The cemetery at Tyne Cot at Ypres, which is the French name for the southern Belgian town of Ieper, is the largest Commonwealth military burial site in the world.
It is a couple of miles from the village of Passchendaele, which gave its name to one of the last battles of WWI.
Tens of thousands of troops were killed or injured in the fighting amid water-filled shell holes.
Readings from some of the great-great grandchildren of servicemen who fought in the conflict, prayers and hymns featured in the service.
One of the most poignant moments came when a vintage bi-plane appeared overhead and dropped poppies - a symbol of the Great War - over the dignitaries.
Peter Cleminson, the Royal British Legion's national chairman, attended the service along with Defence Secretary Des Browne and senior representatives from countries who fought alongside Britain.
He said: "On behalf of The Royal British Legion I would particularly like to thank the people of Ieper, Zonnebeke-Passchendaele and countless other towns in the region who have never forgotten - not for a single day - the hundreds of thousands of men who made the ultimate sacrifice.
"Today we have stood shoulder to shoulder with counterparts from many countries in silence and what I hope was a united spirit of 'never again'."
Rupert Forrester, 18, from Leeds, whose descendants fought near Tyne Cot, said the anniversary had led him to discover a lot about his family ties to Passchendaele.
"My great great grandfather Harry and Ronald, his son, both died on the same day.
"We found the story of how they died and it has meant a lot more to us now."
The men were listed as missing but are both commemorated on Tyne Cot's wall of memorial.
The Queen, dressed in a purple coat and hat, also opened a new visitors' centre at the cemetery.
There are 12,000 graves and 35,000 names of missing people engraved on memorial walls at Tyne Cot.
Before the Tyne Cot ceremony, the Queen laid a wreath at the Menin Gate war memorial, near Ypres, while buglers played the Last Post.
Menin Gate has 55,000 names of missing soldiers engraved on its walls.