A just-published poll says that only 49 percent of the British public can tell you just what D-Day was about. When asked, “Who were the Allied Forces fighting on D-Day?” only 57 percent correctly chose Germany from a list of suggested enemies.

Nearly 1,800 British adults were questioned in the survey, sponsored by the Royal United Services Institute (RUSI) and YouGov. As is often the case, ignorance of what happened was more pronounced among young people, born long after the event. An astonishing one in ten youths questioned in the U.K. Sunday Telegraph in 2014 said they believed Britain had been fighting against France in World War II.

The facts are in the history books, readily available. D-Day (officially known as “Operation Overlord”) was the biggest amphibious invasion in military history, involving some 160,000 American, British and Canadian soldiers, who crossed the British Channel on June 6, 1944, to land on the German-held beaches of Normandy, France.

Of the Americans, 9,388 gave their lives in the invasion. They lie buried at the U.S. cemetery at Colleville-sur-Mer, where visiting President Donald Trump will pay his respects on Wednesday.

The invasion of Normandy marked the turning point of World War II. This offensive led to the ultimate defeat of the greatest evil the modern world has known.

For those who lived through it, D-Day will never be forgotten. This year, Charles Shay, who will turn 95 later this month, was interviewed by The Associated Press about what happened there.

The traumas of that day burned so deep that he did not talk about the experience for well over half a century. But this year he did agree to talk about it.

On the beach, American soldiers were being cut down on every side by German machine-gun fire. Shay started to treat the wounded.

“While I was doing that, I happened to look back out to the water, to the ocean,” he said. He saw many wounded men lying on the beach as the tide was beginning to rise. Without help, they would drown.

“So I dropped what I was doing, and I returned to the water,” Shay said.

The Germans were shooting at any American who moved under their protected Widerstandsnest 62, a bunker that still stands above Omaha Beach. With bullets hitting the sand, Shay started pulling men — he doesn’t know how many — out of the water. Many were much bigger and heavier than he was.

“In such a situation, the adrenaline starts to flow,” Shay said. “It gives you strength that you did not know that you had.” He received the Silver Star for bravery and for saving several of his comrades.

Why did they fight and sacrifice so much on that day?

In fact, they did it for many reasons. They did it because they were ordered to. They did it because they wanted to get the war over with. They did it to end the Nazi tyranny. They did it for themselves and their families.

And they did it for us.

In a world dominated by power and superpower, run on principles of Realpolitik and strategic nuclear weaponry, gratitude seems a thin force in the overall scheme of things.

Yet, 75 years after D-Day, the feeling of gratitude is still strong enough to impel the leaders of the United States and Europe to gather together to remember and pay homage to those who sacrificed life and limb on the beaches of Normandy.

President Trump will be joined by outgoing British Prime Minister Theresa May, German Chancellor Angela Merkel, French President Emmanuel Macron and other European leaders. It’s no secret that today there are issues — like support for NATO — that separate the U.S. and its allies. But the historical memory of Normandy brings them together. The debt to Charles Shay and all the others who fought for freedom transcends these differences.

On D-Day, the sense of gratitude is still strong enough for many of the local French residents to fly American flags as a sign of appreciation for the soldiers who liberated them from four years of Nazi occupation.

In terms of lives lost, D-Day was a terrible thing. Yet, when Charles Shay was asked if it was worth it, he answered: “Oh, yes. Definitely it was worth it. It was a rogue regime that was trying to take over the world, and the people had to be stopped.”

For those who do not know what happened on D-Day, memorial ceremonies like these are important, because they should know. They should know who deserves their gratitude and what to be grateful for.