AMIENS, France (Reuters) - Britain and France on Tuesday marked 100 years since the decisive battle of Amiens that put Europe on the path to the end of World War One, in a sober ceremony in the northern French city’s Gothic cathedral.

British Prime Minister Theresa May and serving army officers read letters and poetry from those who fought in the often-forgotten battle that took conflict out of the trenches and ushered in an armistice 100 days later.

“Amiens was symbolic of the Entente Cordiale, the co-operation without which victory was impossible,” Britain’s Prince William said.

“Today we return to learn more about the experience of those involved during the historic summer of 1918, to honor the fallen of all nations, to commemorate all those who participated in this great endeavor, and to celebrate the bonds of friendship which unite our nations.”

Descendants of soldiers who fought in the battle took part in the ceremony alongside officials from France, Canada, Australia, Ireland and the United States.

May read an extract from the war memoirs of the then-British prime minister, David Lloyd George, which recalled how “the British army itself did not realize the extent and effect of the triumph they had won that day”.

The decisive battle began at 4.20 a.m. on Aug. 8, 1918, when 900 Allied guns opened fire in a surprise attack on German forces.

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Allied troops gained eight miles in one day, captured 450 artillery pieces and 12,000 prisoners -- a huge advance in a war until then characterized by miniscule gains and entrenched stalemate.

Allied forces suffered 19,000 casualties in the three-day battle, while 27,000 German troops were killed or wounded.

At the time, German general Erich Ludendorff described the battle as the “black day of the German army”.

Around 100,000 Australian, British, Canadian and French troops were committed to the offensive.

Among them were British brothers Tom and Robert Slater of the Northumberland Fusiliers. Tom survived, but Robert was killed on the second day of the battle.

Attending the ceremony were Tom’s grandchildren, Dave Slater and Carolyn Gardner, who had only discovered online 10 years ago that their great-uncle had died at Amiens.

“It’s almost a hundred years since our great-uncle passed away. And for me, it’s important to remember the family. It’s important to remember the young men who lost their lives, because it is the ultimate sacrifice,” said Gardner, 58.