President Richard Nixon might have gotten away with it if it weren't for John Dean. In June 1973, Dean testified before Congress that Nixon knew about the Watergate cover-up. Not only that, Dean said he suspected there was taped evidence—and he was he right.

“There are few times in American history where the entire country is focused on one television event,” says James D. Robenalt, a lawyer and author who lectures with Dean about Watergate. “One of them was the Kennedy assassination, one of them was the moon landing, one of them was 9/11, and the other one is John Dean’s testimony. It was that important and that significant.”

Dean was Nixon’s White House counsel on June 17, 1972, the night burglars broke into Democratic National Committee headquarters at the Watergate complex in Washington, D.C. He had no prior knowledge of the break-in or the White House’s involvement. Yet over the next several months, Dean became, as he put it, the “desk officer” for the Watergate cover-up.

“Everybody kind of went through him,” Robenalt says. “He did things like [facilitate payment] to the people who had been arrested to keep them quiet… Which was an obstruction of justice, because they were trying to keep witnesses from honestly and fully testifying before a grand jury about what happened.”

Dean knew that the people receiving payment were involved in the burglary. But he didn’t fully comprehend that he was committing a crime until later, after Nixon won the 1972 election. “One of the burglars called somebody in the White House and just said we’re keeping quiet because of the money that we’re getting,” Robenalt says. “And then it just hit [Dean] right in the face.”

When Dean realized that he was implicated in an illegal cover-up, he didn’t do the right thing immediately. At first, he shredded incriminating files. But on March 21, 1973, he went to the Oval Office and told Nixon there was “a cancer” on the presidency that would take them all down they didn’t stop it. Dean could tell that Nixon had no intention of coming clean, so he decided to, himself.

Before Dean testified before Congress in the Watergate hearings, Nixon called Dean into his office in the Executive Office Building to try and make sure that Dean didn’t implicate him in his testimony. However, his bizarre behavior helped precipitate his downfall.

“You know when I told you we could get a million dollars [to continue to pay the convicted burglars to remain silent] I was just kidding?” Nixon awkwardly asked at that April 15 meeting. Dean said he hadn’t thought that but he’d take his word.

Then Nixon stood up from his chair, walked over to a corner of the office and whispered, “I was wrong to promise clemency for [burglar E. Howard] Hunt when I spoke with Chuck Colson, wasn’t I?” Dean replied, “Yes, Mr. President, that would be considered an obstruction of justice.”

Dean thought it was very weird that Nixon had moved to a different of the room and whispered that question, and he wondered if Nixon had done so because he was secretly taping the conversation and didn’t want that part to be audible. When he testified in June that Nixon had knowingly obstructed justice through the Watergate cover-up, he mentioned this suspicion.

“I don’t know if a tape exists,” Dean said. “But if it does exist…I think this Committee should have that tape because it would corroborate many of the things this Committee has asked me.” Indeed, because other high-level officials lied in their Watergate testimony, the discovery of tapes would be one of the only ways Dean could back up his story about the president’s involvement.

It was just a hunch, but it led to a bombshell discovery. A few weeks later Senate investigators asked presidential aide Alexander Butterfield if he knew about any such tapes, and they couldn’t have picked a better person to question. Not only was Butterfield one of only a few people who knew about the taping system, he was actually the person who helped the Secret Service to install it at Nixon’s request.

“I’m sorry you asked,” Butterfield responded. “But, yes, there was a taping system that taped all presidential conversations.”

The tapes were what brought the whole thing down. Nixon had microphones in the Oval Office, the Cabinet Room, his Executive Office Building office and the Aspen Lodge at Camp David, and also recorded phone calls in the Lincoln Sitting Room. After the Supreme Court ordered Nixon to hand over the tapes to Congress in the summer of 1974, prosecutors found they corroborated Dean’s testimony and implicated the president in the cover-up.

“[Dean] was first and one of the only, actually, in the higher echelons to give honest testimony,” Robenalt says. “Other people who testified, including the chief of staff and the attorney general, they all went to jail for lying about what was going on.”

Dean made a deal where he received a reduced sentence for providing key witness testimony and pleading guilty to obstruction of justice. He served four months in prison and was disbarred from practicing law in D.C. and Virginia.

Still, some of the higher-level Watergate conspirators didn’t actually get a much harsher punishments than Dean. Former Attorney General John Mitchell and former Chief of Staff H.R. Haldeman each served a year and a half in jail for their involvement.

Nixon—the center of the whole scandal—received no punishment at all. He resigned on August 8, 1974 to evade impeachment. One month later, his former vice president, Gerald Ford, pardoned Nixon so that he’d never have to stand trial for his crimes, which were supported by evidence Nixon recorded himself.