WASHINGTON — Fifty years later.

They came from as near as Baltimore, and as far as Tokyo. They came despite swampy heat and intermittent rain. They came braving heat exhaustion to stand in endless lines before the Secret Service checkpoints. They came in memory of lost loved ones. They came in recognition of ongoing struggle. They came in honor of the great man and the great thing he said. Oscar winners and Grammy winners came. Union leaders and activists came. Three presidents came. Janitors and students came. One man came playing “Battle Hymn of the Republic” on his harmonica.

By the tens of thousands, 50 years later, they came. They descended on this city to mark a milestone: half a century since the storied March on Washington.

Fifty is a turning point year in historical commemoration. It marks the moment — invisible, unspoken, but no less real — when a thing begins to depart living memory and to become the exclusive property of history. The man who was an adult when Martin Luther King spoke to 250,000 demonstrators from the steps of the Lincoln Memorial is at least in his 70s now or very near. The clock of life expectancy begins more loudly ticking.

After 50, a thing begins to dwindle in public consciousness. So the 60th anniversary commemoration will be smaller than this, the 70th smaller still, the 80th almost non-existent. If you doubt this, try a thought experiment. Consider the huge national commemoration that did not happen when the attack on Pearl Harbor passed its 70th anniversary two years ago. Consider how April 15th — the date Lincoln died — goes by each year with barely a whisper of acknowledgment.

So this is likely the last time we will do this, the last time we will gather en masse, devote so many front pages, web pages and television hours, to considering the March and the four incandescent words Martin Luther King spoke that day, the words that sealed him in history.

“I have a dream.”

It is, perhaps, difficult for us to truly understand now how much revolution was packed into those words. King, after all, lived in an America where snarling dogs and high-pressure hoses were used to deny the right to peacefully assemble, billy clubs and poll taxes were used to deny the right to vote, lynch rope and explosives were used to deny the right to speak your mind, law and custom were used to deny the right to be. In a word, he lived in a nightmare.

It takes a certain kind of vision to stand inside a nightmare and speak about a dream.

Fifty years later, as we consign that moment to the ages, the commemorative March on Wednesday was about asking a pregnant question, the same question King asked the year before he was killed:

Where do we go from here?

Fifty years later, we have adopted few if any of the policy positions he called for in the book by that name, and that is not likely to change. Movement toward a guaranteed income, for instance, is close to non-existent. That fact notwithstanding, as speaker after speaker noted Wednesday, we have nevertheless “gone” from the world Dr. King knew into one he could hardly have imagined. The nation’s first African-American president made this point simply by walking to a podium that had already been graced by African-American celebrities, politicians, media moguls and dignitaries. He made it, too, in celebrating the marchers who followed King into glory.

“Because they kept marching, America changed,” Obama said. “Because they marched, city councils changed and state legislatures changed and Congress changed and, yes, eventually, the White House changed.”

Regrettably, no Republican official addressed the throng. Regrettably, because concern for issues of civil rights ought not be the exclusive property of any one political party. Though the two Presidents Bush were invited, both declined citing health issues. One Republican, Sarah Palin, did offer her thoughts via Twitter.

“May his dream always be reality,” she said.

“Always be.” Not “Someday become.” It was not a formulation that would have found much favor with the crowd at the Mall on Wednesday, complacently taking for granted as it does that all the work toward equality is already done.

By contrast, if one theme was common among all the speakers and all the crowd, it was that there remains much to do. They seemed to agree that while progress must be celebrated, celebration should not blind us to the fact that equality is not yet reality. In their different ways, they all provided the same answer to that question.

Where do we go from here? We go to work.

“The people united will never be defeated!” chanted the marchers.

I AM A MAN declared a sign, in imitation of the striking sanitation workers King was helping to organize when he died.

Jamie Foxx powerfully challenged his generation of African-American entertainers to emulate the political courage of their forebears.

Bill Clinton called on Americans to no longer “whine” about political gridlock. “It is time to stop complaining,” he said, “and put our shoulders against the stubborn gates holding the American people back.”

U.S. Rep. John Lewis, the only surviving speaker from the 1963 March, noted that though the old segregation signs came down long ago, “there are still invisible signs, barriers in the hearts of humankind that form a gulf between us.”

President Obama challenged African-Americans not to allow bigotry or poverty to be excuses not to raise our children right and teach our children well.

The immortal Shirley Caesar took the congregation to church with a gritty, old-school rendition of a gospel standard whose lyrics were perfect for 50 years later. “My soul looks back,” she sang, “and wonders how I got over.”

Gender equality, the rights of gay men and lesbians, the need to provide a pathway to citizenship for undocumented immigrants, the gutting of the Voting Rights Act, were all recurring themes. Trayvon Martin was there.

And a bell was rung. This was at 3 o’clock, 50 years, to the moment, that King began to speak. It was metaphor turned real, abstraction made solid. Because in his speech, he evoked a bell.

Let freedom ring, said King, “from the prodigious hilltops of New Hampshire .?.?. from the mighty mountains of New York .?.?. from the curvaceous slopes of California.

“But not only that,” he said. “Let freedom ring from Stone Mountain of Georgia. Let freedom ring from Lookout Mountain of Tennessee. Let freedom ring from every hill and molehill of Mississippi. From every mountainside, let freedom ring.”

The bell they used Wednesday was, they said, salvaged from 16th Street Baptist Church in Birmingham, Ala., where, 18 days after the 1963 march, terrorists set off a bomb that killed four little girls in Sunday School.

Wednesday, they struck the old bell and it gave forth a hard, metallic clang that traveled out over that audience of tens of thousands, a mass of people that looked very much like America, black and white and Hispanic and Asian and gay and straight and Jew and Christian and Muslim and man and woman, each with a dream of his or her own.

“Let freedom ring,” said Martin Luther King, 50 years ago.

And for just that moment, it kind of felt like it did.

Leonard Pitts Jr. is a columnist for The Miami Herald. His email address is: lpitts@miamiherald.com.