(CNN) President Donald Trump met the moment, perfectly, with words that summoned poetry from their simplicity.

Turning from a crowd gathered among graves of Americans cut down in Normandy , he gazed into aged faces of their peers who lived and who never forgot the horror of the beaches below . He led a standing ovation to men, many of whom can no longer stand themselves, who had again crossed the ocean -- as they did three-quarters of a century ago on a mission to save liberty.

"You are the glory of our republic and we thank you from the bottom of our hearts," Trump said.

"You are among the very greatest Americans who will ever live," Trump said. "Today we express our undying gratitude."

A group of now hollow-cheeked veterans, wearing medals proudly on their chests, and baseball caps bearing their battle honors, absorbed the applause. Three comrades directly behind Trump's podium huddled under one blanket -- as if in a long-ago fox hole.

Any visit to the killing grounds of Normandy is humbling and inspiring. When world leaders gather to mark historic anniversaries emotions become even more intense. The names, ages and home states of the fallen etched on their graves nearby encapsulates the random horror of a world war.

But there was something different, even more poignant, and final about the D-Day anniversary commemorations this year.

Trump, hitting rare emotional and unifying notes on behalf of Americans and what used to be called the free world, was effectively bidding farewell to the greatest generation.

For once, the divisive rhetoric and dislocation sparked by his approach to the presidency, and the reaction to it from his foes, was forgotten.

There was a sense that a wider cultural and political chapter -- that has helped define life in the West for the last 75 years -- was closing -- leaving a questionable future.

The last of the old guard

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France commemorates D-Day each June, but the biggest events have typically marked five-year intervals, drawing a parade of American presidents to the sacred battlefields and cemeteries.

Each time, the ranks of old soldiers thins considerably. This year, 173 US World War II veterans attended, 65 of those were here on D-Day.

Leaders and politicians who ordered their men into the cauldron of Nazi fire on the morning of June 6, 1944, are long gone. Those that are left to bear witness -- somehow appropriately -- are the foot soldiers who braved the worst of the carnage as they waded ashore in mass ranks to save liberty.

Any 20-year-old who struggled onto Omaha Beach or leapt out of a twin-engine Dakota into a sky boiling with flak that day will be 100 by the time of the next big gathering in Normandy.

A few veterans will likely survive to make the trip, but reality means Thursday's group of nonagenarians represented the last, big pilgrimage to the place where so many of their comrades lie.

Their courage will still be remembered when they are gone -- the gratitude shown by the French towards Americans for their liberation is the glue of the country's relations.

But it will not be quite the same when old men, with their walking sticks and in wheel chairs no longer share their war stories as the trauma of their memories plays out in their eyes.

Trump -- along with French President Emmanuel Macron at the American War Cemetery and other allied leaders at other landing beaches -- were not just saying goodbye to a dwindling army of veterans on Thursday.

They were also marking the end of a pivotal passage in history that is giving way to an age when the institutions forged from the turmoil of World War II are under serious strain.

Trump's speech was a well-judged tribute to veterans and America's allies: The "nobility and fortitude" of the British, the "robust" Canadians, the "fighting Poles" the "gallant French" "intrepid Australians" and "tough Norwegians."

But it offered no larger reflection on the fraught political questions raised by this moment in history -- or about the lessons the D-Day generation may offer for today's world.

The shadow of World War II

It is impossible to overestimate the shadow cast by World War II on the politics and culture of the subsequent age.

D-Day was an especially powerful moment -- it marked the point when America formally succeeded war-ravaged Britain and its fraying empire as the world's top power.

It was a day when a high-risk, logistically complicated multi-national operation with a higher purpose -- the defeat of tyranny -- actually worked against the odds in a show of human ingenuity. It summoned a unity of global purpose that is impossible to imagine in today's fractured politics.

In the years since, some nations have built their entire national self-image around World War II, for better or worse.

"The UK as a nation still identifies itself on as the plucky defender that survived World War II ... against a continental Europe that had been brought under a Nazi yoke," Robin Niblett, director of Chatham House, said on CNN International on Tuesday.

The sense of Britain as a plucky outsider that can fight its own battles and stand alone has been at the center of the Brexit debate that has been replete with wartime references.

It also ignores that the tide was only turned against Nazism when the United States brought its might to bear and after staggering Russian sacrifice bled Hitler's armies in the East.

In America, D-Day and its clear moral frame of a fight between good and evil is a less troublesome national memory than the social and emotional upheaval of the Vietnam and Iraq wars.

World War II in retrospect feels like a sepia moment resonating with national unity that seems impossible to imagine in the angry 21st century.

It is natural that as memories ebb, the lessons of history will also fade. This may explain the rise of right-wing populism in Europe in recent years -- a political tide that was for decades quelled by memories of fascism.

US Presidents and European leaders have also for years been content to reach for easy, unifying narratives of common glory in World War II and the post-Cold War period rather than reboot the transatlantic alliance for the future. Now, they are going to have to try a lot harder since soon, there will be no one alive who remembers the pain and the glory of those wartime years.

For years, the institutions like the United Nations, NATO and the European Union, which the greatest generation sacrificed to build not only ensured prosperity but put a lid on continental conflict that twice in the 20th century drew the United States onto bloody European battlefields.

But now, with the rise of China, as Russia tries to regain lost influence and as an American President, of all people, seems more keen to tear down established structures than to strengthen or modernize them, everything seems fragile.

Macron, addressing US veterans, promised to fight to keep the values for which they fought alive -- despite his own compromised political standing and rising challenges to his internationalist worldview.

"We need to be true to their memory and to do that we must never renounce what their sacrifice ignited," said Macron, who was born 33 years after D-Day.

"The promise of Normandy will be supported by France with all its might. I promise that this will be the case, and this is at the heart of America's destiny, too," Macron said in his own valediction to the greatest generation.