France was the epicentre of the conflict.
Initiated by French President Emmanuel Macron, the Forum aims to revive collective governance, worldwide cooperation, norms, and institutions at a time when global challenges have never been so pressing.
President Trump paid tribute Sunday to USA and allied soldiers killed in World War I as he and dozens of other world leaders commemorated the 100th anniversary of the end of "a terrible, frightful war" that marked America's emergence as a world power. Trump said the comment is "very insulting" and ignored the United States' generous contributions to Europe's defense via the North Atlantic Treaty Organization. "In saying "our interests first and who care about the others" we erase what a nation has that's most precious, what makes it live, what is most important: its moral values".
But Trump was terse during some of his private conversations with world leaders, according to people with direct knowledge of his visit. "So in that sense, I am absolutely a nationalist and I'm proud of it".
President Donald Trump visited the American Cemetery in Suresnes, outside Paris, on November 11, to honor the fallen soldiers of World War I on the 100th anniversary of the armistice that ended the Great War.
Macron, who will deliver a speech in front of the assembled dignitaries, will later host a peace forum at which German Chancellor Angela Merkel gives the opening address.
Trump, who champions a nationalist "America first" policy, will not attend the forum but Putin is expected to.
Rain on Saturday forced the cancellation of Trump's helicopter trip to a different American cemetery in France.
Under a glass canopy at the foot of the Arc de Triomphe, built by Emperor Napoleon in 1806, Trump, Merkel, Macron, Putin and the other leaders listened through earpieces as the French president spoke.
The feminist activist group Femen has claimed responsibility for topless protesters who disrupted President Donald Trump's motorcade on its way to a ceremony commemorating the end of World War I.
French President Emmanuel Macron warned about nationalism during his speech at the ceremony.
Trump came under immediate scrutiny for skipping the trip and was compared to Canada's Justin Trudeau who previous year visited monuments in France under a steady downpour.
Putin, who was last to arrive at the ceremony, gave Trump a brief thumb's up as he greeted them.
On Sunday, however, Macron rebuked Trump's nationalistic tendencies.
Ben Rhodes, who served as deputy national security adviser in the Obama administration, said the White House should have had a fallback plan for the president.
On the other side of the globe, Australia and New Zealand held ceremonies to recall how the war killed and wounded soldiers and civilians in unprecedented numbers and in gruesome new, mechanized ways.
As he was arriving, a woman ran out toward the U.S. presidential motorcade with slogans - including the word "fake" - written on her chest.
U.S. Army Capt. Charles Normington wrote that "each soldier had his arms full of French girls, some crying, others laughing; each girl had to kiss every soldier before she would let him pass". At the war's end, the US had 2 million troops in Europe and another 2 million ready to cross the Atlantic if needed, a force that turned the United States into a major military power whose soldiers then fought and died again for Europe in World War II.