The Memorial Day parade in Washington in 2017. (Michael S. Williamson/The Washington Post)

IN MANY communities, including Washington, people will march Monday in warm local tributes to the many thousands who have given their lives for their country. Later this year, there may be, by presidential order, a much grander parade, inspired by the Bastille Day celebration in France during President Trump’s 2017 visit. It’s the sort of military spectacle this country hasn’t staged for a long time, perhaps because the most powerful nation on Earth felt it unnecessary.

But there have been parades that carried an emotional charge for Americans. Among the most famous, immortalized in scores of photographs and documentary films, was the march along the Champs-Elysees by thousands of American soldiers, their ordered ranks filling the whole broad boulevard after the liberation of Paris in 1944 . The thing that needs to be remembered when pondering the faces of those soldiers is that this was not the climax to a happy story for them. The Parisians would return to their homes; the Americans and their allies would keep marching east into the heart of Germany, against ferocious resistance. Thousands of them would die.

In a democratic country, it takes a deep and widespread sense of obligation to wage that sort of struggle. People can be, and were, compelled to serve, but there are limits to what a decent government can do to enforce compliance with its laws — especially by those who are morally opposed to killing or even, as in one famous Supreme Court case, to reciting the Pledge of Allegiance. In the end, the vital component of a free people’s defense is a sense of obligation, set forth at the U.S. Military Academy but understood by all who serve: Duty. Honor. Country. These are not Twitter words. They are engraved in our national consciousness.

Last week Mr. Trump presumed to suggest professional football players who don’t stand for the national anthem ought to consider leaving the country. In so doing, he shows, again, that he does not understand what truly makes America great.

He should (for once) consult history for some perspective. David W. Blight, a Civil War historian at Yale University, has written about what he regards as the first Memorial Day. It came on May 1, 1865, less than a month after the surrender at Appomattox, when thousands of former slaves and other African Americans in Charleston, S.C., pooled their efforts to give proper burial to several hundred Union soldiers who had been held as prisoners of war at a racetrack and had died there under atrocious conditions. The field was covered with flowers for the occasion, and schoolchildren and women’s groups marched and sang patriotic songs, including “We’ll Rally Round the Flag” and, of course, the national anthem. It was a spectacular display of love of country, and of hope for the future.

“They were themselves the true patriots,” writes Mr. Blight. But, he adds, “Despite the size and some newspaper coverage of the event, its memory was suppressed by white Charlestonians in favor of their own version of the day. From 1876 on, after white Democrats took back control of South Carolina politics and the Lost Cause defined public memory and race relations, the day’s racecourse origin vanished.”

There’s much to remember on Memorial Day — and to learn.