COLLEVILLE-SUR-MER, France—The first thing you notice, at the end of the narrow roads that lead to this precipice, is how peaceful this place is. The cliffs are thick with rough green vegetation and drop down—sharply, then more gradually—to a Prussian-blue sea and a windswept beach. Omaha Beach.

The morning I went, the sun was bright, and a few people were walking on the sand with a dog. I could see them from a lookout on the pathway to the Normandy American Cemetery here, where more than 9,300 servicemen and a few servicewomen are buried—neat rows of milk-white marble crosses, 150 Stars of David, and 307 graves of unknown dead that read, simply, “Here rests in honored glory a comrade in arms, known but to God.”

I had been told nothing quite prepares you for this place, and it was true.

June 6 is the 75th anniversary of the D-Day landings. In the past, American presidents have used D-Day to mark a moment—from Ronald Reagan, who gave his “Boys of Pointe du Hoc” speech at the 40th anniversary in 1984, at the peak of the Cold War, to Barack Obama, who addressed the 9/11 generation of veterans at the 70th in 2014.

This year’s commemoration, though, will likely have a different tone. Donald Trump, who will attend a ceremony here with French President Emmanuel Macron, has been threatening—in words as powerful as actions—the solidarity and mutual understandings of NATO. He’s been lashing out at Europe, accusing it of trying to rip off the United States, which has provided for the Continent’s defense since the Second World War.

But something else will be different too: Ceremonies are held every five years, and this will likely be the last time D-Day veterans will attend. It’s hard not to see this year’s ceremony as the end of a cycle of history—one that began with the Allies, led by the United States, turning the course of war here in Normandy and ended with the president of “America First,” who has made questioning the transatlantic alliance a pillar of his presidency.

“I always liken D-Day at 75 to 1938 in Gettysburg,” says Robert Dalessandro, the deputy secretary of the American Battle Monuments Commission, which maintains the cemetery here and many others around the world. In 1938, when the Civil War was just barely close enough to touch, President Franklin D. Roosevelt went to Gettysburg to inaugurate a memorial there. Invoking Abraham Lincoln, he spoke before veterans of the North and the South about how he was “thankful that they stand together under one flag now.” At the time, FDR knew that Civil War veterans would not live much longer, Dalessandro told me, adding, “In my heart, I know this is the last time we’re going to get D-Day veterans to this ceremony.” (This year, he said, about 35 D-Day veterans will attend.)

I had taken a strange, solitary pilgrimage here to begin to comprehend what had happened on these beaches in June 1944 after General Dwight D. Eisenhower—briefed on the weather and the winds and the tides and the tiny window in which such a massive, complex operation of air and sea and land and 150,000 troops could take place—gave his now-famous response, after less than a minute of reflection: “Okay,” he said, “we’ll go.”

And so they went. Anyone who has ever traversed this stretch of Normandy, the coast as it winds its way from Omaha Beach west toward Cherbourg, and understood the topography—the cliffs and the precious few routes inland meant the Allies couldn’t bomb heavily without risking being stranded on the beach—anyone who has ever set foot here comes away with two questions: How did these men pull this off? And what would have happened if they hadn’t?

Consider how many tens of thousands of individual decisions were necessary to achieve the cumulative effect of breaching the German defenses so enough Allied troops could pour into Europe to defeat the Nazis. At Omaha, the troops traversed more than 200 yards of beach, then had to scale 35 to 60 yards of cliffs, all while under enemy fire. Before the invasion, the Allies organized a misinformation campaign: Phantom field armies were planted; a group in Dover, England, tried to trick the Germans into thinking the invasion would be in Pas de Calais, not Normandy; an actor was hired to impersonate British General Bernard Law Montgomery to throw the enemy off course; planes dropped metal strips to scramble German radar, and leaflets telling the Germans their army was in retreat when it wasn’t.

Martha Gellhorn, the indefatigable reporter, managed to make the English Channel crossing to Normandy on a hospital vessel. She wrote about the chaos of the D-Day invasion, of the medical triage on the ship, and of small human moments in the middle of battle. “After that there was a pause, with nothing to do. Some American soldiers came up and began to talk,” she wrote. “Everyone agreed that the beach was a stinker and it would be a great pleasure to get the hell out of here sometime. Then there was the usual inevitable comic American conversation: ‘Where’re you from?’ This always fascinates me; there is no moment when an American does not have time to look for someone who knows his hometown.”

In all, about 225,000 service members were killed or wounded or went missing in Normandy from June to August 1944, including 134,000 Americans and 91,000 Britons, Canadians, and Poles, as well as 18,000 French civilians. The Germans lost more than 400,000 soldiers in Normandy. In La Cambe, inland from Utah Beach, I visited a German cemetery. The graves of the 21,000 dead are marked with flat stones, interspersed with a few clusters of thickly hewn crosses. A sign in English and French reads: “With its melancholy rigor, it is a graveyard for soldiers not all of whom had chosen either the cause or the fight. They too have found rest in our soil of France.”