“We had to choose a night. It was clear there was a big night coming up on March 8. It was the biggest fight night of the year, of the decade.” — John Raines, “1971”

Not that John Raines; his wife, Bonnie; and the rest of their Citizens’ Commission to Investigate the FBI were boxing fans, anyway. Even for what was being billed as the “Fight of the Century” on that Monday night 50 years ago: the first meeting between undefeated world heavyweight champion Joe Frazier and the indomitable Muhammad Ali.

But the commission, composed of antiwar protesters in the Philadelphia area, needed the perfect day, the perfect time, the perfect moment to make its task as perfectly inconspicuous as possible.

“We talked about the weather, the day of the week,” Keith Forsyth, one of the members of the group, told me last week from his Philadelphia home. “Then somebody — I don’t know who, probably Bill [Davidon] — said a lot of people, including the cops, would be listening to that fight, and that would give us cover.”

What March 8, 1971, is most remembered for is that epic prizefight. It stopped the country in its tracks and the world, it seemed, on its axis. Ali was the most famous athlete on Earth and possibly its most loquacious human. He had become a face and voice of the anti-war movement. The previous year, he had beaten in court the boxing officials who in 1967 stripped him of the title Frazier acquired, as well as his license to fight, for refusing conscription into the Vietnam War. Frazier was an establishment figure who supported the war.

Swashbuckling billionaire sports team owner Jack Kent Cooke — of the Washington football franchise and the Los Angeles Lakers — put up $4.5 million of the fight’s record $5 million purse. He sold its broadcast rights in dozens of countries but not in the United States, where he prohibited networks from showing or even describing the action. Instead, you could watch it live only at a few hundred theaters and arenas that sold tickets for closed-circuit screenings.

You also could buy one of the 20,455 seats at Madison Square Garden, but they sold out in a few hours to the rich and famous. “Plumage, pimps and hustlers,” fight publicist Bobby Goodman described it to Sports Illustrated. Who was there? Woody Allen. Diana Ross in black velvet hot pants, Bill Cosby, Barbra Streisand. Harlem drug kingpin Frank Lucas. Joe Namath. Hugh Hefner’s girlfriend Barbi Benton, with a monkey-fur coat covering a see-through blouse. Paul Newman. Sen. Ted Kennedy.

The Apollo 14 astronauts — including Alan Shepard, the first American in space — returned from the moon just in time to sit ringside and look up at the stars. Former vice president Hubert Humphrey only got an upper-deck seat. Frank Sinatra received a photographer’s ringside pass, arrived with a 35mm camera and got a corner perch, with his photographs later appearing in Life magazine.

Yet it’s what the citizens’ commission dared to attempt — and somehow pulled off — that lives on as more than a mere memory from that day. It was the ultimate use of sports for political action, and it changed America. It would reveal, among other things, how Black Panthers leader Fred Hampton was assassinated by the FBI and Chicago police, as dramatized in “Judas and the Black Messiah,” the biopic for which Daniel Kaluuya (as Hampton) just won best supporting actor at the Golden Globes.

As the glitterati sashayed into the Garden half a century ago, the commission members, wearing secondhand clothes, assembled at a motel about a 30-minute drive west of Media, Pa. Its members included the late John Raines, a Temple University religion professor and former Freedom Rider; his wife, Bonnie, who had been inspired by the civil rights movement; and Forsyth, who hitchhiked to Philadelphia in 1970 after leaving the College of Wooster early to join demonstrations against the Vietnam War. “I was a fan of Ali,” Forsyth said, “when he said no Vietnamese ever called me a you-know-what.”

Davidon, a physicist who had worked at the Enrico Fermi Institute and the Argonne National Lab and became increasingly opposed to nuclear proliferation and the war, was the group’s strategist. There were four more, including two whose names have remained secret. As former Washington Post reporter Betty Medsger chronicled in her 2014 book, “The Burglary: The Discovery of J. Edgar Hoover’s Secret FBI” — and Johanna Hamilton documented in her film “1971” — the members planned to commit a felony by breaking into an FBI office on a downtown corner of Media. They would put themselves at risk of imprisonment or worse.

It wasn’t unusual at the time for protesters to burglarize government offices that were seen as cogs in the war effort, particularly draft boards, where they could destroy files and at least temporarily impede the effort to snatch young men into the military. But the commission’s aim was different. It sought to expose what many in the antiwar movement thought was the government’s fight against those opposed to the war.

“Citizens who are serious about protecting our democracy are going to be activists, hopefully,” Bonnie Raines told me last week. “That’s why we had to do that extreme action. Because no one in Washington was going to hold Hoover accountable. No one. So then it’s the job of the ordinary citizen to take action.”

Around 8 p.m. that day, when the commission believed the Ali-Frazier fight was to start, Forsyth drove from the motel to Media, dressed in a thrift-shop Brooks Brothers suit. He parked near the building housing the FBI office, grabbed his briefcase with locksmith tools he had taught himself to use and walked up to the second-floor office, where he was to spring the door open. But he was surprised to find an extra lock installed that Bonnie Raines, who surveilled the office during business hours pretending to have interest in an FBI job, hadn’t reported.

“We had to scrap the whole plan,” Forsyth told me.

So he hustled back to the motel and reported the problem.

Time was of the essence. But while monitoring the proceedings at the Garden on the radio, the commission realized the main fight hadn’t started and wouldn’t until after 10. So the group decided Forsyth should return to the FBI office and use a crowbar to pop the lock. The building’s caretaker lived on the first floor, directly below the FBI office.

“From the caretaker’s apartment, I could hear crowd noise [from the fight],” Forsyth told me. “My idea was one big noise was better than a lot of small noises. So I waited for a crowd roar and then gave [the crowbar] a big shove.”

It worked. But something heavy was behind the door.

“I didn’t want to knock it over,” Forsyth said. “So I decided to slide it.”

Inch by inch he went, hoping every roar of the crowd would muffle the noise he was making. Finally, he made it — and notified the quartet of commission members who were to clean out the office.

“The four who went into the office said they could hear [the fight] on peoples’ radios in the apartments in that building,” Bonnie Raines told me. “They said it was a good thing that the fight was later. Because it was right at the right time to be in the office.”

Bonnie Raines remained outside, around the corner, standing in front of her car with the hood up, pretending to be in distress to attract a police officer’s concern should any drive by. None did.

“I think it took the people, the four who were inside the office, not more than an hour to scoop up all the documents,” she said.

Then it was off to a farmhouse outside Media to sort through the papers.

“Everybody knew the FBI was not our friend,” Forsyth said. “But when you find the details, it knocks you out.”

They copied the documents on a Xerox machine and mailed them to a senator, a congressman and major newspapers. Medsger at The Post was one recipient; The Post was the first newspaper to publish the documents.

The most startling revelation was of a 15-year-old program called COINTELPRO, short for Counter Intelligence Program. It spied on civil rights leaders such as Martin Luther King Jr., anonymously threatening to expose his extramarital affairs if he did not kill himself. It spied on Ali as early as 1966. It paid Hampton’s security chief, William O’Neal, to inform the FBI about what Hampton and the Black Panthers were up to, ultimately leading to the assassination of Hampton and Mark Clark, another leader of the group.

All of it was laid bare in a mid-1970s Senate hearing and investigation. COINTELPRO was shut down, and an FBI spokesman told the New York Times when Medsger’s book was published that “a number of events during that era, including the Media burglary, contributed to changes to how the FBI identified and addressed domestic security threats, leading to reform of the FBI’s intelligence policies and practices and the creation of investigative guidelines by the Department of Justice.”

“John and I were invited to the Black Panthers’ 50th anniversary in Oakland, which was a wonderful occasion,” Bonnie Raines said. “But they said one of the reasons it was so important that we be there was that, they said, ‘If it wasn’t for the Media burglary, a lot more of us would be in prison now.’ ”

Frazier defeated Ali that night. I remember sitting in the kitchen with Mom and Dad, listening to round-by-round reports from the Garden and being stunned into disbelief when the announcer said Frazier knocked down Ali.

“I wasn’t really thinking about the fight at the time,” Forsyth said of his moments jimmying his way into the FBI office. “But I remember at some point being disappointed that Ali lost.”