The Rolling Thunder caravan of motorcycles rolled into Washington Sunday, to support the U.S. military on the country's Memorial Day weekend, but it turned into a political rally for the presumptive Republican presidential nominee, real estate titan Donald Trump.

As thousands of motorcyclists cheered their support, Trump vowed, "We have to rebuild our military. It's been decimated."

He said that if he is elected, "We're going to knock the hell out" of Islamic State jihadists in the Mideast.

But Trump made no mention of one of the controversies from early in his months-long run for the Republican nomination, when he said the 2008 Republican presidential nominee, Senator John McCain, was not a hero for being captured as a prisoner of war in Vietnam in the 1960s.

More recently, Trump, who avoided military service with a series of deferments, has had to answer questions about why he had not met a pledge to send $6 million to veterans groups after a fundraiser in January. He told the military veterans on Sunday that he will name the groups receiving the money this week.

Participants drive toward the Lincoln Memorial dur FILE - Participants drive toward the Lincoln Memorial during the Rolling Thunder 'Ride for Freedom' in Washington, May 25, 2014. (Brian Allen/VOA) FILE - Participants drive toward the Lincoln Memorial during the Rolling Thunder 'Ride for Freedom' in Washington, May 25, 2014. (Brian Allen/VOA)

The annual Rolling Thunder event, first organized in 1988, is billed as a tribute to prisoners of war and Americans missing in action, a day ahead of Memorial Day Monday, when the United States honors its war dead.

Ray Manzo, a former U.S. Marine Corps corporal, is one of the men credited for organizing the Rolling Thunder event. A letter from Manzo was published in Outlaw Biker magazine in 1987 calling for riders to descend on Washington the Sunday before Memorial Day.

Nearly 3,000 riders came for that first ride. The organization says the event has now grown to more than a million riders and spectators combined. A half million motorcycles are expected to rumble Sunday afternoon from the Pentagon to the Vietnam War Memorial for a rally in an event the group calls a Ride for Freedom.

The group's mission has grown alongside the ride. The organization lobbied for the Missing Service Personnel Act of 1993, which prohibits the U.S. Defense Department from declaring a service member killed in action without substantial evidence.

It has also established a nonprofit organization that provides cash aid to veterans' families.

Ken Caryl of Dale City, Virginia salutes the motor FILE - Ken Caryl of Dale City, Virginia salutes the motorcyclists, many of them are veterans, as they ride across the Memorial Bridge into Washington during the annual Rolling Thunder "Ride for Freedom," May 26, 2013. FILE - Ken Caryl of Dale City, Virginia salutes the motorcyclists, many of them are veterans, as they ride across the Memorial Bridge into Washington during the annual Rolling Thunder "Ride for Freedom," May 26, 2013.

Over the years, Rolling Thunder has become a more politicized group frustrated with Washington lawmakers and the administration of President Barack Obama.

"I don't think anyone in Washington, D.C. really cares about investigating the live POWs that the government knows were left behind," writes Sgt. Artie Muller, Rolling Thunder's national executive director, and Joseph Bean, the group's national president, in a letter posted on the Rolling Thunder website.

The letter, dated December 2015, also says "This administration keeps getting us deeper and deeper in the war against Islam . . . Why are we not demanding reparation payments from the Islam countries for the expense of what these wars cost the America taxpayers....We better pray for a new administration in the 2016 election that will follow the Constitution of the United States and worry about America, our people and our troops first."

Motorcycle enthusiasts have frequently attended Trump campaign rallies to show their support for the candidate they believe is different from career politicians they accuse of degrading the military and ignoring veterans.

At a political event last July when asked about McCain, who was described by the questioner as a "war hero,' Trump responded “He’s not a war hero. He’s a war hero because he was captured. I like people that weren’t captured, O.K.?”