It begins in the summer of 1968. Nixon feared a breakthrough at the Paris Peace talks designed to find a negotiated settlement to the Vietnam war, and he knew this would derail his campaign.
He therefore set up a clandestine back-channel involving Anna Chennault, a senior campaign adviser.
At a July meeting in Nixon's New York apartment, the South Vietnamese ambassador was told Chennault represented Nixon and spoke for the campaign. If any message needed to be passed to the South Vietnamese president, Nguyen Van Thieu, it would come via Chennault.
In late October 1968 there were major concessions from Hanoi which promised to allow meaningful talks to get underway in Paris - concessions that would justify Johnson calling for a complete bombing halt of North Vietnam. This was exactly what Nixon feared.
The US delegation, left, and North Vietnamese delegation at Paris peace talks The Paris peace talks may have ended years earlier, if it had not been for Nixon's subterfuge
Chennault was despatched to the South Vietnamese embassy with a clear message: the South Vietnamese government should withdraw from the talks, refuse to deal with Johnson, and if Nixon was elected, they would get a much better deal.
So on the eve of his planned announcement of a halt to the bombing, Johnson learned the South Vietnamese were pulling out.
He was also told why. The FBI had bugged the ambassador's phone and a transcripts of Anna Chennault's calls were sent to the White House. In one conversation she tells the ambassador to "just hang on through election".
Johnson was told by Defence Secretary Clifford that the interference was illegal and threatened the chance for peace.
President Nixon in 1970 with a map of Vietnam Nixon went on to become president and eventually signed a Vietnam peace deal in 1973
In a series of remarkable White House recordings we can hear Johnson's reaction to the news.
In one call to Senator Richard Russell he says: "We have found that our friend, the Republican nominee, our California friend, has been playing on the outskirts with our enemies and our friends both, he has been doing it through rather subterranean sources. Mrs Chennault is warning the South Vietnamese not to get pulled into this Johnson move."
He orders the Nixon campaign to be placed under FBI surveillance and demands to know if Nixon is personally involved.
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