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Newly unsealed grand jury testimony by ex-President Richard Nixon shows he warned prosecutors and grand jurors not to probe an episode from 1971, when he discovered that the Joint Chiefs of Staff had been spying on him and national security adviser Henry Kissinger.

“Don’t open that can of worms,” Nixon told his interrogators in June 1975, when he spent roughly eleven hours over two days’ time fielding – and sometimes deflecting – questions put to him by lawyers for the Watergate Special Prosecution Force and two grand jurors flown in from Washington.

The long-secret session – which marked the first time any president had given grand jury testimony – was held at a Coast Guard facility adjacent to Nixon’s property in San Clemente, California. It has long intrigued historians because Nixon, having been pardoned by his successor, Gerald R. Ford, in September 1974, never testified elsewhere in detail about Watergate.

The 278 pages of testimony were unsealed Thursday pursuant to an order by a federal district judge in Washington, who had granted an unusual petition by Watergate historian Stanley Kutler and the group Public Citizen. The Department of Justice, under Attorney General Eric Holder, had formally opposed the papers’ release, citing grand jury secrecy rules, but opted not to appeal the judge’s decision.

Nixon’s testimony on the Pentagon spying case, which he mentioned only fleetingly in his memoirs, represents his most extensive comment on the episode in any forum – and even today, six pages remained classified, redacted in full by the National Archives. The ex-president explained in detail his January 1972 decision to order a wiretap on Navy Yeoman Charles Radford, a young stenographer detailed to Kissinger’s National Security Staff.

For thirteen months across 1970 and 1971, Radford later admitted, he had rifled Kissinger’s briefcase and wastebaskets and delivered up to 5,000 classified documents to his Pentagon superiors, who in turn shared the materials with the chairman of the Joint Chiefs of Staff at the time, Admiral Thomas Moorer. Radford was also determined to have leaked some documents to syndicated columnist Jack Anderson, who won a Pulitzer Prize for his columns based on the top-secret papers.

Nixon was first informed of the affair in December 1971, and reacted with shock, terming it “a federal offense of the highest order.” His initial instinct was to prosecute Moorer for espionage, but then-Attorney General John N. Mitchell swayed the president against that course of action, citing the adverse news coverage such a prosecution would generate. Nixon also followed Mitchell’s advice to transfer the yeoman to Oregon, where members of the twenty-seven-year-old stenographer’s family lived.

“I said, ‘Ship him out,’” Nixon testified. “But it was vitally important that he be tapped to see whether this mania he had developed for leaking was continuing, and so he was tapped and his closest associates were tapped for about six months. They were knocked off in June [1972], on June 20, when the tapping was concluded…I would strongly urge the special prosecutor don’t open that can of worms, because there is even more, because [Radford]…was a direct channel to the Joint Chiefs of Staff.”

On Watergate, Nixon reaffirmed what historians now generally believe: that he did not order the infamous break-in that triggered his fall from power, and did not have any advance knowledge that it would be carried out. “I learned about [the break-in] in Florida,” the ex-president testified. “I saw [White House Chief of Staff H.R.] Haldeman there and we rode back on the plane together…We discussed this to be terribly wrong, and also in my opinion utterly stupid activity.”

Nixon also testified extensively on the eighteen-and-a-half-minute gap that was discovered on the tape of his June 20, 1972 conversation with Haldeman, held just three days after the Watergate burglars were arrested. Nixon’s longtime secretary, Rose Mary Woods, acknowledged that she might have caused part of the erasure during her transcription of Nixon’s tapes. “I don’t know how it happened,” the ex-president said. “If you are interested in my view…it was an accident. My view as far as Miss Woods’s role is that… anything she did, it was an accident…[perhaps] some malfunction of the machine.”

At times, Nixon sparred testily with his interrogators, sometimes scolding them for improper procedure or questioning their impartiality. “I am not unaware of the fact,” he snapped at one point, that “you would have a motive to go after me and my associates and to ignore others.” “Don’t put words in my mouth and make me lie about something,” he barked at another point. “I am not going to be loose with my tongue and try to cooperate with you in a vendetta.”

Recalling the Pentagon Papers case, when Daniel Ellsberg, a former protégé of Kissinger’s, leaked 7,000 pages of classified documents about the Vietnam War to the New York Times, Nixon testified: “Dr. Kissinger said that Ellsberg…was a ‘nut,’ that he was unstable, therefore untrustworthy, and he didn’t know what he might do.”

Determined to have his say on a broad array of topics, Nixon let fly disparaging words for a few of his favorite targets: Jack Anderson, the “intellectual and emotional eunuchs” at the State Department, Kennedy patriarch Joseph P. Kennedy, and Martha Mitchell, the outspoken, eccentric wife of the attorney general (“How he stood that woman that long, I will never know”).

And he confided what his predecessor in the Oval Office, Lyndon B. Johnson, lamented during their closed-door meeting at the White House in December 1968, during the transition between their administrations. “He told me, very emotionally, that the greatest mistake he made was after his election in his own right in ’64, in not firing all of the people or virtually all of the people whom he had inherited from the [Kennedy] administration and getting his own people in.”

Also released by the National Archives were forty-five minutes of recordings Nixon made on his Dictabelt machine, separate from his infamous taping system. Nixon used the Dictabelt to dictate memoranda for Haldeman and also to set down his account of his late-night talk with students at the Lincoln Memorial in May 1970, shortly after the killings at Kent State University.

The Archives also unsealed an undated recording from the spring of 1973 by Haldeman aide Gordon Strachan. Talking with fellow Haldeman staffer Lawrence Higby, Strachan can be heard discussing the role of White House Counsel John Dean in the disbursement of “hush money” to the men arrested at Watergate. The conduits Dean used included Strachan and Nixon campaign aide Fred LaRue. “I took 40 [thousand] to LaRue, because Dean told me to take 40 [thousand] to LaRue,” Strachan said. “It would be, you know, the same night that Dean told me to do it.”

James Rosen is Fox News’ chief Washington correspondent, and author of The Strong Man: John Mitchell and the Secrets of Watergate.