COLLEVILLE-SUR-MER, FRANCE — Onofrio Zicari had never been able to bring himself to return to the beaches of Normandy.

But this year, at 96, the retired Los Angeles milkman decided he had to come back to the place seared into his memory from the morning of June 6, 1944, when he stormed Omaha Beach in the fifth wave of incoming soldiers on D-Day. He flew from his home in Las Vegas to northern France — nearly 5,300 miles — to find one particular white cross in the American cemetery.

Donald E. Simmons was the last one out of the landing craft that morning, as Zicari and the others made their way across the water and through an onslaught of German gunfire from a ridge in the distance. Simmons was killed almost instantly, Zicari said, his hand on his friend’s grave. “He was my buddy.”

At 21 and 20, Zicari and Simmons were still boys on D-Day. They would have had a hard time imagining the world 75 years later. Only one of those boys lived to see the end of the war, the rites of marriage and fatherhood, the grandeur of what was called the American century.

Hovering above a foreign shoreline, the cemetery presents a particular image of the United States abroad. This is a memorial to a proudly internationalist society that — to quote the inscription on the memorial chapel here — sacrificed its sons “for the common cause of humanity.” But, 75 years later, America’s role on the world stage no longer seems as certain. The future of the postwar order won in battles such as D-Day is anyone’s guess.

President Trump campaigned — and won — on the creed of “America First,” a catchphrase that evokes an America entirely foreign to the beaches of Normandy and that, in any case, Zicari was uninterested in discussing. “I don’t like to get into politics,” he said.

When asked why he came back, he said: “So the nightmares would stop.”

For presidential historian Jon Meacham, D-Day is a symbol whose meaning has changed with the times — in the mid-1960s, it was a Cold War rallying cry; in the mid-1980s, an underpinning of Ronald Reagan’s call for American restoration.

This was the essence of Reagan’s 1984 “Boys of Pointe du Hoc” speech (credited to speechwriter Peggy Noonan). “And I tell you, we are ready to seize that beachhead,” Reagan said.

In 2019, Meacham noted, the beaches of Normandy have yet another meaning.

“This year, I think many Americans who are likely to be sentimental about the story of Operation Overlord are also likely to be supporting a president whose instincts are isolationist, not interventionist, and who takes a dim view of the postwar order that more or less kept the peace for more than half a century,” he said.

“These beaches teach us the steep toll of isolation and America First — and should be perennial reminders that we cannot escape history.”

Pierre Vimont, a former ambassador to the United States, said the isolationist rhetoric emanating from the White House does not accurately reflect the status of the transatlantic relationship enshrined in Normandy.

“Despite the sometimes spectacular declarations, the foundations of this relationship remain solid,” Vimont said. “There is a reality of cooperation and transatlantic relations that remains very strong.”

But other European observers point to genuine transatlantic divergences that transcend, and even predate, the theatrics of Trump’s Twitter account: the U.S. “pivot to Asia,” the feeling that Europe should shoulder more of its own defense burden, the list goes on.

“Nostalgia can’t guide us,” said Benjamin Haddad, a French political scientist and head of the Atlantic Council’s Future Europe Initiative.

“The very fact that we have peaceful, stable democracies in Europe despite the challenges they faced is an incredible testimony of American success, but this is also why Europe will be less central to America in the 21st century than it was in the 20th, and we should celebrate that,” Haddad said.

“The question is how do we still keep a positive agenda, still understand that our bonds are stronger than what divides us, but at the same time not be in denial about the very real disagreements we have,” he said.

To wander the pathways of the manicured cemetery in Colleville-sur-Mer — with 9,388 white crosses and Stars of David extending as far as the eye can see — is to contemplate the cruel specter of the what-might-have-been. The things undone, the lives unlived.

Zicari made clear that everyone who experienced D-Day left something on these beaches, even those who survived. In coming back, he said he hoped for only one thing.

“I’m 96 years old. And my kids said, ‘Go ahead, Dad. You’ll have your closure.’ ”

Alice Crites in Washington contributed to this report.