"We're gonna keep riding until we get everybody back home, from all wars," said Jack Richardson, who at 73 crossed the country from California for the 13th time to participate in the annual Memorial Day weekend spectacular.

Dressed in a leather jacket emblazoned with patches, this Vietnam War veteran had assembled with thousands of other bikers in a parking lot near the Pentagon, awaiting the start of the parade.

The route will take them into the center of official Washington, past the monuments on the National Mall and the austere black marble memorial engraved with the names of the nearly 60,000 US soldiers killed during the Vietnam War.

"Still there are families waiting back home here in the United States that have not found out where their dads, their fathers, their brothers -- they don't know where they are," said Richardson, a retired Los Angeles police officer who served two tours in Vietnam in the 1960s.

"They don't know if they're still in Vietnam, they don't know if they're still alive, they don't know if they're dead, they don't know if they're captured," he said.

According to organizers, more than 85,000 US soldiers remain unaccounted for in conflicts as far back as World War I.

Most are from World War II, but 1,598 of the missing are from the war in Vietnam, a conflict still fresh in the memories of older veterans.

The parade was begun in 1988 with some 2,500 motorcycles under the motto "We will never forget" to press for an accounting of the Vietnam missing.

It has grown every year since into a rumbling, roaring extravaganza that organizers say attracts over a million people, including spectators.

Besides the missing, the bikers also come to remember their fallen comrades.

That's what motivated Mel Goudge, who served in Vietnam from 1965-66.

He is taking part in Rolling Thunder this year for the first time at age 76, after a 10-day cross-country ride from his home in Washington state.

"The ride was hard on me," he said. "But I'm glad I did it."

"It was a pilgrimage. I grew up with a friend who lived across the street and he was shot down in 1968, November 25th, and I needed to come to the wall and touch his name on the wall. It was kind of something I had to do," he said.

Reminiscing about the war as they sip coffee amid American flags, most avoid talk about politics, but Donald Trump won many over by visiting Rolling Thunder during his 2016 presidential campaign.

The US president tweeted his appreciation ahead of the parade.

"Fantastic to have 400,000 GREAT MEN & WOMEN of Rolling Thunder in D.C. showing their patriotism. They love our Country, they love our Flag, they stand for our National Anthem!"