Could you define more specifically what you are looking for? Do you mean in general what Kissinger said was Nixon's downfall, or are you referring to a specific book, speech or article that Kissinger wrote? Kissinger has written many works that discuss the Nixon administration, Watergate, the Viet Nam War, etc., so it would help to have more information.
We all know that Watergate - the break-in ordered by Nixon of the Democratic headquarters that was located in the Watergate Hotel in Washington - led to a series of events that unraveled the Nixon presidency and led to his resignation in 1974. Nixon feared he would not be re-elected for a second term, so he sent the so-called "plumbers" to break into the Democratic headquarters to obtain information that might give him an edge in his campaign. I have read that Henry Kissinger often stated he believed the Viet Nam War was a "catalyst" that led to Watergate. Nixon feared the continued unpopularity of the Vietnam War would lead to his defeat and this fear made him so paranoid that he would do anything to give himself an advantage.
If you read about these two men - Kissinger and Nixon - you will find that they really did not like or respect each other very much. They called each other names behind each other's back, but in their own ways, they needed each other. Kissinger was one of the few in Nixon's administration not to be indicited for some crime associated with Watergate.
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