The US did not resort to using nuclear weapons in Vietnam for a variety of reasons: fear of the damage it would cause to the US's international reputation, domestic political considerations, a reluctance to break the 'tradition' of non-use, and a realization that, although there were plenty of viable targets such as airfields, ports and supply lines, only extensive use of nuclear weapons would be likely to have a decisive military impact.
Added to this was the strong opposition on moral grounds from key figures such as Secretary of Defense Robert McNamara, and President Johnson's concern at the long-term consequences of the use of such weapons.
A 1966 CIA Memorandum for the Director came out strongly against the use of nuclear weapons in Vietnam for a variety of reasons. Principle among these were that there
would be widespread and fundamental revulsion that the US had broken the 20-year taboo on the use of nuclear weapons.
Another section adds weight to the above point:
Their use in Vietnam, regardless of the circumstances, would send a wave of fear and anger through most of the informed world.
The report also mentions that there
would be intense agitation in Japan, probably leading to a restriction US use of Japanese facilities and possibly to denunciation of the US - Japan defense treaty...[and] a probably resolution of condemnation in the UN; and a marked diminution of such public support as US policy in Vietnam now has.
The report also says that the use of nuclear weapons might lead to a Chinese withdrawal but
we think it more likely that they would not do so
At the same time, though, the author(s) believed that the USSR would not use nuclear weapons or otherwise become directly involved. Instead, the Soviets would exploit the US action for propaganda purposes.
Among other possible or likely negative consequences, the report mentions
- "NATO would be badly shaken"
- European support would evaporate and the British government would fall if it did not condemn the US
- the US would be the main target of calls to disarm while, at the same time, there would be nuclear proliferation because some countries would feel the need the acquire nuclear weapons.
There were other opinions, though. Tannenwald notes that, in a report in May 1967, the Joint Chiefs raised the possibility of using nuclear weapons in southern China, a prospect described by General Robert Ginsburgh in a September 1967 memo as "virtually unthinkable" when he was deputy to national security advisor Walt Rostow.
Less than a year later, the US commander in Vietnam, General William Westmoreland approved a planned contingency operation called Fracture Jaw which recommended the use of tactical nuclear weapons in January 1968:
Westmoreland cabled to Admiral Sharp a recommendation that the Commander in Chief, Pacific (CINCPAC), and MACV begin contingency planning for the use of tactical nuclear weapons in northern Quang Tri if necessary to prevent a major defeat. He noted that in the uninhabited mountains around Khe Sanh, such weapons could be used with great effect and with “negligible” civilian casualties.
When the White House found out, which it inevitably had to as only the President could approve the transfer of nuclear weapons to Vietnam, the plan was quickly shut down. In a memorandum to President Johnson, Defense Secretary Robert McNamara stated that the use of nuclear weapons was not an option
because of terrain and other conditions peculiar to our operations in South Vietnam, it is inconceivable that the use of nuclear weapons would be recommended there against either Viet Cong or North Vietnamese forces
Rumours of the proposed use of nuclear weapons had also become public by February 9th:
By that time...the issue had become public in the United States, with Senator Eugene McCarthy and others charging that the military was preparing to use nuclear weapons in South Vietnam. The administration, facing a domestic and foreign outcry, publicly disavowed any such intent.
Further, according to Tom Johnson who was then "a young special assistant to the president and note-taker at the meetings on the issue"
“When he [the President] learned that the planning had been set in motion, he was extraordinarily upset and forcefully sent word through Rostow, and I think directly to Westmoreland, to shut it down,”...
He said the president’s fear was “a wider war” in which the Chinese would enter the fray, as they had in Korea in 1950.
Johnson had already gone on record as being opposed to the use of nuclear weapons. In a speech on Labor Day 1964, he said:
For 19 peril-filled years no nation has loosed the atom against another. To do so now is a political decision of the highest order. And it would lead us down an uncertain path of blows and counterblows whose outcome none may know. No President of the United States can divest himself of the responsibility for such a decision.
Source: Chapter 6 in Nina Tannenwald, 'The Nuclear Taboo: The United States and the Non-Use of Nuclear Weapons Since 1945' (2008)
Johnson was, at the time, campaigning against the 'pro-use of nuclear weapons' Barry Goldwater but then United States Under Secretary of State McGeorge Bundy said LBJ's speech wasn't just politics:
Bundy wrote later that although there was politics in Johnson’s speech, there was “passionate conviction” as well. Two factors appeared to be key in Johnson’s thinking: the long-term effect of any use of the bomb “on the survival of man” – a prudential consideration – and the desire not to be the first president in twenty years to use nuclear weapons, that is, to break the powerful “tradition” of non-use that had now developed – a taboo consideration.
Source: Tannenwald
It wasn't just the people at the top of the administration such Johnson, Bundy and McNamara who were against the use of nuclear weapons:
Most scientists and civilian defense analysts involved in policy advising opposed use of nuclear weapons in Vietnam, for both military and moral reasons.
Source: Tannenwald
As an example of the above, see this March 1967 report http://nautilus.org/VietnamFOIA/report/dyson67.pd f" rel="nofollow noreferrer">Tactical Nuclear Weapons in Southeast Asia. One exception to this opposition was the physicist Samuel T. Cohen who, although defiant, wrote that:
anyone in the Pentagon who was caught thinking seriously of using nuclear weapons in this conflict would find his neck in the wringer in short order.
Source: Samuel Cohen, 'The Truth About the Neutron Bomb' (1983)
Tannenwald summarizes the decision to not use nuclear weapons in Vietnam thus:
Several considerations motivated non-use of nuclear weapons in Vietnam: the possibility of inadvertent and uncontrolled escalation with the consequences this entailed for US vulnerabilities, preservation of the tradition of non-use, and finally a taboo, a normative belief that using nuclear weapons would be wrong. For many US leaders, nuclear weapons were morally repugnant. To be militarily decisive, such weapons would probably have to have been used in large numbers, and this would have been politically and normatively unacceptable.
