Richard Nixon, Traitor
So, seems like Nixon sabotaged Lyndon Johnsons attempts at peace talks with Vietnam in order to further his presidential candidacy:
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.
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.
I'd claim I was surprised, but y'know, it's Nixon. Kind of makes me wonder how many other presidential candidates have been ruthless enough to pull something like that though.