As the doors open on Monday for the second day of the Paris Peace Forum, the image of United States President Donald Trump attending a ceremony to mark 100 years since the end of the First World War on Sunday is hard to forget.
The US president sat mostly stony-faced as he listened to his French counterpart, who sees himself as Europe's foil to the rising nationalist sentiment which has taken hold of so many countries.
President Emmanuel Macron delivered a stinging indictment of nationalism in a 20-minute speech and called on leaders to learn the lessons of the past.
"Nationalism is a betrayal of patriotism," he said.
Mr Trump, who proudly declares himself a "nationalist", stood apart, even on a continent where his brand of populism is on the rise. Neither he nor Russian President Vladimir Putin joined other leaders in a symbolic walk up the Champs-Elysees to the Arc de Triomphe and the Tomb of the Unknown Soldier.
The visit made clear that, nearly two years after taking office, Mr Trump has dramatically upended decades of American foreign policy posture, shaking allies.
After the ceremony, German Chancellor Angela Merkel and UN Secretary-General Antonio Guterres gave the opening speeches at the new peace forum in Paris, set up by Mr Macron.
In a continent devastated by two world wars, leaders like Mrs Merkel warned that rising nationalism poses a threat to peace.
"The concern I have is that blinkered nationalist views may gain ground once again," Mrs Merkel said, warning that people were calling the "European peace project" into question.
Mr Guterres warned of "many parallels with both the start of the 20th century and the 1930s, giving us grounds to fear that an unpredictable chain of events could ensue".
The second day of the forum will have discussion panels on various ways to ensure international co-operation in tackling global challenges and in ensuring durable peace. Governments, NGOs, companies and experts, among others, will gather under one roof for three days to debate how best to find a solution to today's transborder challenges.
The earlier show of unity among some of the world leaders at the Arc de Triomphe comes at a time of growing tensions between liberals and populists in the EU, as well as between Europe and the US, two years into Mr Trump's "America First" presidency.
Up to 1,500 anti-Trump protesters held a demonstration at Place de la Republique square in eastern Paris on Sunday, at which the giant "Baby Blimp" balloon depicting the US president as an infant was flown, following previous appearances in London and New York.
"Mr Hate, Leave Europe," read one banner at the protest.
President Trump snubbed the Peace Forum, instead visiting a military cemetery in the Paris suburbs, a day after coming under criticism for cancelling a similar trip due to rain.
Mr Putin meanwhile, said he and Mr Trump — at odds over Washington's sanctions and the abandonment of a Cold War-era nuclear treaty — had a brief but positive conversation during the centenary commemoration events.
The Kremlin said earlier that the prospect of a full meeting between the US and Russian presidents had prompted huge international media interest, leading to concern from the French organisers that this could overshadow the commemorations.