US President Joe Biden warned that democracy is once again under threat in a speech to commemorate D-Day in Normandy, France, on Thursday.
"We're living in a time when democracy is more at risk across the world than at any point since the end of World War II," he said at the American Cemetery in Colleville-sur-Mer.
"Isolationism was not the answer 80 years ago and is not the answer today."
D-Day was the largest amphibious assault in history. It laid the foundations for the Allied victory over Nazi Germany in World War II, with Biden calling it a "powerful illustration of how alliances, real alliances make us stronger."
Britain's King Charles III and Canadian Prime Minister Justin Trudeau also attended the memorials in Normandy, representing the major Allied powers that took part in the military operation.
"We recall the lesson that comes to us, again and again, across the decades: free nations must stand together to oppose tyranny," Charles said.
They were joined by around 200 surviving veterans.
Biden draws parallels with Ukraine
Biden used the opportunity to underscore Washington's support for Ukraine in the face of Russian aggression.
"We will not walk away because if we do Ukraine will be subjugated and it will not end there," Biden said.
"Ukraine's neighbors will be threatened, all of Europe will be threatened."
Biden described Russian President Vladimir Putin as a "tyrant bent on domination."
'I was proud of myself': D-Day veteran tells his story
"To surrender to bullies, to bow down to dictators, is simply unthinkable," he added.
"If we were to do that, it means we'd be forgetting what happened here on these hallowed beaches."
European leaders thank veterans
World leaders attended a number of other events held on both sides of the English Channel to honor those who sacrificed their lives 80 years ago.
At a ceremony for Canadian soldiers held on Juno Beach in Normandy, French Prime Minister Gabriel Attal said his country's gratitude was "eternal."
"France will never forget those who died for her," he said.
Meanwhile, French President Emmanuel Macron awarded 11 US veterans and one British veteran with France's highest distinction, the Legion of Honor.
"You are back here today at home, if I may say," he said in English to the veterans, who are now aged between 98 and 104 years old.
German Chancellor Olaf Scholz said the soldiers' courage paved the way for freedom and democracy in his own country.
"Their courage gave us Germans the chance of a new beginning," he wrote in an article published in the Ouest France newspaper.
zc/nm (AP, Reuters, AFP)