Merkel and Macron have emerged as the leading defenders of the multilateral institutions created at the end of World War II, while Trump, who embraces what he calls "America First" policies, pronounced at the United Nations in September that he's a "nationalist" who rejects globalism.
Standing in a Paris drizzle just feet from leaders including Presidents Donald Trump and Vladimir Putin, Macron used the culmination of a ceremony commemorating the centenary of the end of World War I to champion the work of global institutions such as the European Union and the United Nations.
"It is our duty to preserve the civilization they defended and to protect the peace they so nobly gave their lives to secure one century ago", he said.
Trudeau has had to navigate the mercurial American president, and talked pointedly about him on Sunday afternoon, but never mentioned Trump by name, as he is prone to do. She had slogans, including the words "Fake" and "Peace", written on her chest.
The service concluded with the bugle call that was played at 11.00 am on November 11, 1918 to signal the end of fighting on the Western Front.
Macron said "nationalism is a betrayal of patriotism".
For her part, German Chancellor Angela Merkel has made an impassioned plea for world peace and cooperation 100 years after Germany's defeat in World War I, saying "we must not simply stand by and watch" as more conflicts continue to unfold around the world.
Mr Trump, addressing a crowd seated under shelter from the light drizzle, strayed from his speech and said: "You look so comfortable up there under shelter as we're getting drenched, you're very smart people".
Minutes before the start of the ceremony two topless protesters from the radical feminist group Femen attempted to waylay Trump's motorcade as it rolled up the Champs-Elysees.
He skipped out of a visit to an American military cemetery outside of Paris yesterday due to a threatened rainstorm, and did not walk with other world leaders in France today.
Trump spoke during a WWI centennial at the Suresnes American Cemetery and Memorial where more than 1,500 US military servicemen are buried.
Asked in late October what he meant when he described himself as a nationalist, Trump cited his love of the country and said it was about putting it first economically.
But Macron told CNN: "To be very direct with you, what I don't want to see is European countries increasing the budget in defence in order to buy American and other arms or materials coming from your industry".
But on Sunday both the USA and Russian presidents arrived separately for the solemn ceremony at the Arc de Triomphe, skipping the bus ride and symbolic walk with other leaders. "We have a lot of problems".
The Forum takes place in the midst of the commemoration of the 100th anniversary of the signing of the Armistice, which marked the end of World War I.
The President's chief of staff, John Kelly, and Chairman of the Joint Chiefs of Staff General Joe Dunford traveled to the cemetery to lay a wreath instead.
Mr Trump did not attend, and left Paris on Air Force One shortly after the forum began. Its role as host of the main global commemoration aimed to highlight the message that the world must not stumble into war again, as it did so quickly and catastrophically with the second world war.