Thirty one years ago, Vietnam vets Artie Muller and Ray Manzo agreed they were plenty disturbed by news reports that suggested Americans were still being held as prisoners of war. They also agreed it was time to draw attention to the issue, and to “never forget” POWs, those who never made it home and fellow veterans who were struggling to adjust to civilian life.

“We were ordinary men who understood that they had a right to have their voices heard,” said Mr. Muller in his own description of that moment in 1987 which led to the creation of Rolling Thunder, named for the fierce bombing campaigns over North Vietnam.

The pair called for a demonstration in the nation’s capital to draw attention to those issues — and one so loud that even politicians would hear it. In an era before social media and the internet, their message was too powerful to be ignored. Close to 3,000 motorcyclists showed up for the first Rolling Thunder “Ride for Freedom.”

The event now draws 750,000 riders from every state in the union and a dozen foreign nations. It is the world’s largest gathering of motorcycles — and one fueled by their cause and a political calling which has not lost its intensity.

Former Presidents George W. Bush and Barack Obama, along with President Trump have all met personally with Mr. Muller and his closest Rolling Thunder officers, as have multiple members of Congress. Mr. Bush, in fact, regularly received the group in the White House driveway during his time in office, and was happy to accept an honorary black leather vest from the organization.

Lawmakers, along with military brass and Cabinet members have made the ride themselves. As governor of Indiana, Vice President Pence rode with Rolling Thunder. Former Alaska governor Sarah Palin has a well. The message, however, remains paramount.

“What is our government doing to recover remains from crash sites and battlefields? When is someone going to take action and put the live POW/MIA issue out front at a meeting with all foreign countries involved in past wars?” asked Mr. Muller. “We must never give up and let our government forget about the issue. American troops will always face the enemy in foreign wars, for the freedom of others.”

Rolling Thunder, the organization, is run entirely by volunteers in 90 chartered chapters across the nation. Anyone can join, a motorcycle is not required. The civic-minded organization also stages local and regional rallies to raise money through a separate charitable division for disabled and homeless vets, rehabilitation centers, military families in need and senior citizens.

The ride itself begins on Sunday at noon on a route that takes the riders along the Potomac River from the Pentagon to Vietnam Memorial, in a dedicated horde so vast that it takes six hours to assemble them all in formation. American flags are everywhere.

Once they get going, the actual Rolling Thunder ride takes about 30 minutes,and is followed by a public program on the National Mall which has drawn coverage from all major cable and broadcast networks, along with C-SPAN.

The experience for riders and onlookers can be profound. The event has won praise for its patriotism, organization and the sheer scope of it.

“The camaraderie of the bikers, their generosity of spirit, and the rolling thunder of their engines was unique and awe-inspiring,” one witness advised in an online review on TripAdvisor.com.

“Truly humbling and what a much needed show of patriotism for the men and women who put their lives on the line to protect us. Why isn’t there more press coverage of the event? God bless our troops and those who organize this,” another wrote.

There’s a new Rolling Thunder anthem as well, written and sung with gusto by Rockie Lynne, a chapter vice president who hails from North Carolina who will perform the song after the ride.

“You can feel them coming, from all across the nation, chrome on steel, hell on wheels — to say what needs saying. Like pilgrims dressed in leather, the faithful get together,” the lyrics advise.