Theresa May, Prince William commemorate 'Entente Cordiale' in Amiens
The descendants of soldiers who took part in the Allied push, which smashed German defenses and morale, traveled from across the world to Amiens for the ceremony in the city's magnificent medieval gothic cathedral.
Senior officials from Britain and France were joined by representatives from the Australian, Canadian and US governments in honor of the tens of thousands of troops killed in the four days of fighting.
Former German president Joachim Gauck and Scottish leader Nicola Sturgeon also attended.
The Battle of Amiens sounded the start of the Hundred Days Offensive on the Western Front, which led to the Armistice in November 1918.
The Allies deployed hundreds of tanks to push deep into German lines on what German General Erich Ludendorff called a "black day" for the German army.
May's visit was her second to France in under a week, coming days after she held talks with President Emmanuel Macron at his Mediterranean holiday retreat over her Brexit plan.
Macron, a native of Amiens, did not make the trip to his hometown for the event, where France was represented by Armed Forces Minister Florence Parly.
May read from the war memoirs of British wartime leader Lloyd George, who said that the gains made in the northern French city led the Germans to "realize that all hope of victory had passed".
William, May, and Gauck also spent time talking to soldiers' families.
Denis Holden, a 65-year-old retired policeman, traveled from Melbourne in Australia in memory of his grandfather, Private Michael Willis of the 14th Australian Infantry Battalion, who was injured in shelling on the first day of battle.
"He never told us anything about the war but he showed me the wounds down his back," Holden told AFP.
He was also scarred psychologically.
"In the years after the war, whenever his Model T Ford backfired, he would yell out 'Get under the bed, we'll all be killed!'. I guess that today you'd call it post-traumatic stress syndrome," said Holden, who wore two of his grandfather's service medals pinned to his blazer.
"It's very important to keep everyone's memory alive," said Clarke, who wore two remembrance poppies pinned to her lapel.
By the summer of 1918, the US was pouring troops into France and the Allies had drastically boosted their firepower after four years of war that had already killed nine million soldiers on three continents.
A victory at the Second Battle of the Marne in July 1918 emboldened the Allies to strike at German forces which had exhausted themselves in a massive offensive meant to finish the war earlier in the year.
When guns began pounding German positions in Amiens -- a key rail hub -- at 4:20 a.m. on August 8, the Germans were caught off guard.
Marooned in dense fog, many dazed soldiers surrendered to the Allies, who punched a gaping, 12-kilometer hole into German lines, backed by some 600 tanks and 2,000 warplanes.
The stunned Germans, who had 27,000 troops killed, injured or captured on the first day, never recovered. By early September they were in retreat and two months later the conflict was over.
But two decades later many of the officers who made a name for themselves in the so-called "war to end all wars" were back in action.
Attending today's ceremony was Helen Patton, granddaughter of World War II hero General George Patton.
World War 1 was a "training ground" for Patton, who was badly injured in fighting near Verdun in September 1918.
"He learned to pray, to cuss and to handle troops", his granddaughter said.
"War is often glamourized but not World War 1" Helen Patton lamented. "It's as if we almost wanted to forget it."
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