Nixon also sought help from Chiang Kai-shek, the president of Taiwan. And he ordered Haldeman to have his vice-presidential candidate, Spiro T. Agnew, threaten the C.I.A. director, Richard Helms. Helms’s hopes of keeping his job under Nixon depended on his pliancy, Agnew was to say. “Tell him we want the truth — or he hasn’t got the job,” Nixon said.

Throughout his life, Nixon feared disclosure of this skulduggery. “I did nothing to undercut them,” he told Frost in their 1977 interviews. “As far as Madame Chennault or any number of other people,” he added, “I did not authorize them and I had no knowledge of any contact with the South Vietnamese at that point, urging them not to.” Even after Watergate, he made it a point of character. “I couldn’t have done that in conscience.”

Nixon had cause to lie. His actions appear to violate federal law, which prohibits private citizens from trying to “defeat the measures of the United States.” His lawyers fought throughout Nixon’s life to keep the records of the 1968 campaign private. The broad outline of “the Chennault affair” would dribble out over the years. But the lack of evidence of Nixon’s direct involvement gave pause to historians and afforded his loyalists a defense.

Time has yielded Nixon’s secrets. Haldeman’s notes were opened quietly at the presidential library in 2007, where I came upon them in my research for a biography of the former president. They contain other gems, like Haldeman’s notations of a promise, made by Nixon to Southern Republicans, that he would retreat on civil rights and “lay off pro-Negro crap” if elected president. There are notes from Nixon’s 1962 California gubernatorial campaign, in which he and his aides discuss the need to wiretap political foes.

Of course, there’s no guarantee that, absent Nixon, talks would have proceeded, let alone ended the war. But Johnson and his advisers, at least, believed in their mission and its prospects for success.

When Johnson got word of Nixon’s meddling, he ordered the F.B.I. to track Chennault’s movements. She “contacted Vietnam Ambassador Bui Diem,” one report from the surveillance noted, “and advised him that she had received a message from her boss … to give personally to the ambassador. She said the message was … ‘Hold on. We are gonna win. … Please tell your boss to hold on.’ ”