He also paid his respects to the thousands of American soldiers who died in WWI: "It is our duty to preserve the civilization they defended and secure the peace they so nobly gave their lives for one century ago".
Elsewhere, ceremonies in New Zealand, Australia, India, Hong Kong and Myanmar began a day of remembrance services around the world for a conflict that involved millions of troops from colonised countries in Asia and Africa.
The "America First" leader, who faced criticism on Saturday for cancelling a trip to an American cemetery because of rainy weather, will snub the Peace Forum.
On Sunday, around 70 world leaders, including Trump, Russia's Vladimir Putin, Turkey's Recep Tayyip Erdogan, and Merkel, are due to attend the main centenary ceremony at the Tomb of the Unknown Soldier under the Arc de Triomphe.
Canadian Prime Minister Justin Trudeau (left) and U.S. President Donald Trump attend a First World War armistice anniversary ceremony at the Arc de Triomphe in Paris on November 11. And Trump was not present as the other leaders arrived, walking side-by-side in a somber, rain-soaked line holding black umbrellas as bells finished tolling.
The ceremony will mark the very hour - 11 am on November 11, 1918 - when the guns fell silent after more than four years of slaughter on the Western Front. She had slogans, included the words "Fake" and "Peace", written on her chest. The woman was tackled by police and the motorcade continued uninterrupted. The Femen group claimed responsibility.
French President Emmanuel Macron had said on Friday that Europe should build a "true European army" so the continent can protect itself from Russia, China - and the United States. Merkel said close worldwide cooperation with a common set of values is the only way to overcome the horrors of the past and build the future.
French President Emmanuel Macron and German Chancellor Angela Merkel hug after unveiling a plaque in a French-German ceremony in Rethondes, France, on November 10, as part of commemorations marking the 100th anniversary of the November 1918 armistice, which ended World War I. The two leaders seek to preserve a West-led social democratic order that they feel is threatened both at home and overseas.
Meanwhile, Macron has repeatedly emphasised the need for a global order and the rejection of nationalism. And you know what? "So in that sense, I am absolutely a nationalist and I'm proud of it".
United Nations chief Antonio Guterres meanwhile warned of "parallels" between the present day and the unstable and unsafe 1930s as he marked the centenary of World War I in a speech at the Paris forum. "We have a lot of problems".
The Russian president is hoping his personal ties with Trump, who regularly touts their good relationship, will at some point yield a softening of the USA approach to his country.
As the other leaders gathered in the morning, Trump tweeted about the devastating wildfires in California, criticizing local officials for poor forest management.
"Nationalism is a betrayal of patriotism", he said.
The two leaders were scheduled to sit next to each other at a lunch, but France changed the seating plan at the last minute.
The President has accordingly directed Foreign Minister Gbehzongar Findley to utilize diplomatic channels to ensure opportunities for deeper bilateral discussions with those leaders in the near future.