Daniel Ellsberg is most famous as the military analyst who leaked "The Pentagon Papers" to the American press in 1971, revealing top secret information about US planning and decision-making in regard to the Vietnam War.
In regard to the question about October of 1969, the Nixon administration was at a point where they wanted to break the stalemate in Vietnam. According to Ellsberg, Nixon believed he could end the war by threatening the use of nuclear weapons against Vietnam, and had actually identified targets for nuclear strikes. Again, according to Ellsberg, the huge Moratorium to End the War in Vietnam demonstration, which included teach-ins and demonstrations all across the United States, convinced President Nixon not to increase the war in that way -- not to threaten nuclear war.