American adults of a certain age grew up learning about D-Day, which occurred 75 years ago June 6 along a French coastline. And while the saga remains in high school history books, many young people today can't identify what happened in the early 20th century. They may forget the grave sacrifices made for world freedom.

That shouldn't be allowed to happen, ever.

On that fateful day, American, French and British troops made up the crux of a force that landed at Normandy, with about 5,000 ships and 13,000 aircraft involved in the assault. More than 9,000 soldiers in the Allied invasion group either died or were injured, and they were realistic about their chances: They knew they might not survive. But they figured their sacrifice would help ensure an ultimate victory over Adolph Hitler and the original "Axis of Evil." They were right, as the invasion paved the way for more than 100,000 troops to slowly drive into Europe.

Thanks to the 9/11 nightmare, Americans know what it feels like to targeted, as a nation, by terrorists. But Europeans have been used to that for a long time. Because of two world wars, Europeans also understand the horrors of actual military campaigns, attacks and bombings on their own soil. That is something with which U.S. citizens are wholly unfamiliar. It's why Europeans have a more pragmatic and cautious attitude than Americans, and why they are offended when Americans seem at times to be spoiling for war.

Queen Elizabeth understands all too well the toll war takes on human beings. At 93, she must have vivid memories of what happened during World War II, because at the time of D-Day, she would have just reached adulthood. She, like the dwindling numbers of American veterans who served in that campaign and others, does not want another war like that to happen again.

That likely explains her gift to President Donald Trump during the first day of his visit to London: the abridged first edition of "The Second World War," by Winston Churchill. Trump has expressed admiration for Churchill in recent days, though it's unlikely the late former British prime minister would have thought much of the current U.S. president. Churchill was, after all, known for expressing a preference for pigs over dogs or cats, and at times, people. He explained that while dogs look up to humans and cats look down on them, pigs treat us as equals.

Mr. Trump should find the time to read this book. If he did, he might pause before offering up rhetoric that suggests to many Europeans he's inviting a conflict. It's possible that he, like some others of his generation, still views the pursuit of conquest with admiration – a way for an aging statesman to cement his legacy. That wouldn't be the case here, in Europe, or anywhere else – except perhaps among the entourage of Vladimir Putin, who would love to see Russia returned to its former glory.

D-Day is not an occasion to be celebrated, but rather commemorated – much like Memorial Day, just a little over a week ago. Those who admire Mr. Trump for his spunk should keep their fingers crossed that he approaches this day with the solemnity it deserves.