Video report by ITV News Correspondent Romilly Weeks
Theresa May has paid tribute to the "courage, bravery and skill" of troops who fought in the First World War Battle of Amiens as a poignant commemoration service marked its centenary.
Relatives of soldiers who served and died in the conflict also spoke of their pride and sadness as they joined the Prime Minister and the Duke of Cambridge at the event staged exactly 100 years since the start of the offensive.
The battle changed the course of the war, as the comprehensive Allied victory, due to superior tactics, use of technology and leadership, finally convinced German commanders they could not win.
William acknowledged the debt owed to the First World War troops in a message printed in the official programme: "The Battle of Amiens, and the continued fighting which followed during the summer of 1918, brought the Allies hope and optimism after four long years of bloodshed and stalemate.
"While it is right that we have collectively commemorated many of the significant battles and campaigns of those years, it is important that the success of the Battle of Amiens takes its rightful place in our shared history."
During his address to the congregation, William said: "What began here on August 8 was truly a coalition operation under the strategic command of a great Frenchman, Marshal Foch, a battle in which the forces of many nations came together to fight; in which aerial, mechanical and human courage and ingenuity combined with devastating results.
"Amiens was symbolic of the Entente Cordiale, the co-operation without which victory was impossible.
"It is entirely fitting therefore, that today, that same international coalition has returned to Amiens with our former enemy in peace and partnership."
Kevin Sherlock, 58, a retired Rolls-Royce engine inspector from Derby, said his 19-year-old great uncle Ernest Harm, who was killed towards the end of the battle, "paid the ultimate sacrifice", helping to bring peace to Europe.
He said the teenage soldier, also from Derby, a Lance Corporal in the King’s Own Yorkshire Light Infantry, 2nd Battalion was killed, possibly by machine gun fire, going "over the top" on August 11 1918.
Mr Sherlock added: "I think as I’ve got older I’ve thought about it more really, he was only 19, just a lad really, never experienced anything in his life.
"So there’s sadness but also pride that he took part in a key battle that brought ultimately the Allied victory and no doubt about it, shortened the war as well."
Commenting on the fact his great uncle has no known grave Mr Sherlock added: "It must be very painful for a mother to come to terms with, she probably thought he was listed as missing, killed in action, but there’s always a small part of you thinking he might walk through the door one day."
In Amiens Cathedral in northern France, the story of the battle was told through contemporary letters, diaries and poems read by guests from the 2,000-strong congregation.
Among those invited were Armed Forces minister Mark Lancaster, his French counterpart Florence Parly, Chief of the Defence Staff General Sir Nick Carter, who is head of Britain’s Armed Forces, and representatives of other nations including Germany.
In her message printed in the official programme Mrs May highlighted how the battle of Amiens heralded the beginning of the period known as the Hundred Days offensive.
After the 1918 conflict successive military victories eventually led to the surrender of German forces and the end of the conflict on Armistice Day on November 11 that year.
The Prime Minister, who has been holidaying in Europe, wrote: "Today, we commemorate that success, but we also reflect on the fear and hardship experienced by the people of this city and the surrounding battlefields, as well as the immense suffering demoralisation of the German troops.
"We remember with profound respect all those who served on both sides of the battle and we give thanks for their courage, bravery and skill which would lead to what the world had long yearned for, the guns finally falling silent."