It's been 79 years since D-Day landings. How experts say we'll continue to honor WWII veterans

Seventy-nine years ago, on June 6, 1944, more than 156,000 Allied troops invaded Normandy, France, on D-Day.

"It was one of the most momentous events of the last century," April Cheek-Messier, president and CEO of the National D-Day Memorial Foundation, told ABC News. "The men and women of D-Day and World War II literally saved the world. They saved the world for the next generation."

As nations reflect on Operation Overlord, the mission that changed the course of history and liberated Europe from Hitler's Germany, America also faces down another fact: The number of WWII veterans, especially those who participated in D-Day, is dwindling.

Experts say those WWII veterans who are no longer with us would want their stories passed on, and it's critical to honor their legacies and preserve history by doing so.

"We need to educate [people] on the stories we do know and pass them on. They devoted their lives to that," military historian, author and professor John C. McManus told ABC News, echoing something he said during a D-Day commemoration last year: "The reality is, once that generation is gone, which it almost entirely is, it's incumbent on historians to carry on that legacy."

The Normandy landings represent something "monumental," he said, and it's hard to imagine anything more important than telling WWII veterans' stories.

The National D-Day Memorial Foundation was originally about paying tribute to those who made the ultimate sacrifice on June 6, 1944, and the tens of thousands who died in the weeks that followed, Cheek-Messier said.

It also communicated to veterans that "we know what you did, why your story is important, why we need to pass it on," she said. "We wanted to make sure we paid tribute to those veterans. Their stories had not really been told."

In addition to maintaining the monument, which is more than 20 years old and located in Bedford, Virginia, the foundation also undertakes the difficult research of confirming the names of those who died on D-Day.

When the foundation was formed, there was no list, database or roster that existed, Cheek-Messier said. Now, the foundation maintains the most complete name-by-name listing in the world of Allied service members who died on June 6, 1944.

According to the foundation's necrology database, 4,415 men died that day, including 2,502 Americans.

"Remembrance is key. It's central to any lasting memorial," Cheek-Messier said. "Education is central to remembrance."

She said veterans have a desire for their stories and memories to be passed on to future generations: "Many of them came home. Many of their friends did not."

As the foundation looks toward the 80th commemoration of D-Day next year, the research and education continue.

"How are we as a nation now remembering? How are we passing on their stories?" Cheek-Messier said.

The foundation expects to be able to add some new names to its wall at the Bedford memorial in 2024, she noted, so those individuals can also be recognized.

"Now that we're nearing the 80th anniversary, we realize most are no longer with us," she said. "We can't hear these stories firsthand. We have to make sure we're able to tell their stories. Not just us at the memorial, but all of us as American citizens."