Why did the United States not resort to nuclear weapons in Vietnam?

Probably also because the Non-Proliferation Treaty (NPT) was signed on 1968-07-01 and came into effect on 1970-03-05¹. It was negotiated during the 1950s and 1960s.

The signatory states of the treaty with nuclear weapons (and the USA is one of these) have made it clear that they will not use nuclear weapons against signatory states without nuclear weapons except in response to a nuclear attack, or a conventional attack in alliance with a Nuclear Weapons State.

So unless the USA wanted to throw the NPT into the dustbin and risk nuclear proliferation (and nobody wants that), they could not use nuclear weapons against North Vietnam (or Cambodia or Laos).

Note that we are normally talking about usage of nuclear weapons as actually detonating them. That's forbidden and nobody has done that since 1945. But of course they have been used by openly or covertly threatening their detonation. Daniel Ellsberg has reported in The Doomsday Machine that the USA did that on multiple occasions because it's policy on first use isn't as clear as one would hope.

¹ When did the USA ratify the NPT?

Question: Why didn't the United States resort to nuclear weapons in the Vietnam War.

Simply put that was one of the questions decided in the 1964 election where the conservative Barry Goldwater seemingly favored the use of nuclear weapons in Vietnam.

• Goldwater said: "By one impulse act you could press a button and wipe out 300 million people before sundown."
• Goldwater said: "Commanders in Vietnam already had the authority to use nuclear weapons and did not need a presidential authorization."
• Goldwater also said: "I would remind you that extremism in the defense of liberty is no vice! And let me remind you also that moderation in the pursuit of justice is no virtue!"

How Daisy Add Changed Everything about Political Advertising

The incumbent president Johnson capitalized on these statements by airing the famous Daisy Attack Add, Sept 7, 1964. The Johnston campaign aired it only once and was pulled from the airwaves it probable aired an additional 1000 times free of charge by the media, the Johnston campaign had no need to pay to run it again. Its shocking point was made. A vote for Goldwater was a vote for nuclear war.

Lyndon Baines Johnson went on to win the election by the widest majority since James Monroe's virtually uncontested second term election of 1820.

From the Comments

@Ed999 The Democrats won that election in '64 because their incumbent President had been assassinated 12 months earlier, not because of any perception that Barry Goldwater was pro nuclear war. Goldwater could have campaigned on any issues he cared to, or not, and nothing would have affected the outcome. That election turned solely on the issue of the assassination, and nothing was ever going to influence the result, whoever the Republican candidate had been or whatever policies the GOP had espoused.

That's a bit of an oversimplification and ignores President Johnson's incredible transformation and political astuteness which brought about his landslide victory in the 64 election. Johnson the son of a poor sharecropper from west Texas was a lifelong segregationist who was considered unelectable to national office because of his long history championing the policies of the South.
Johnson transformed himself into President Kennedy's torch bearer beginning the day after the assassination. In his first televised national address Johnson threw his whole weight and the emotional outpouring for the slain president behind the stalled 1964 Civil Rights Bill. Personally stewarding it through congress.
In championing a Kennedy signature initiative which Kennedy had been unable to pass through Congress himself, Johnson solidified himself with Kennedy's political machine, and the nation, and eliminated his greatest impediment to winning the Presidency on his own.
Yes, portraying Goldwater as out of touch and willing to use nuclear weapons in Vietnam went a long way to painting Goldwater as unelectable in the minds of the American voters. Both of these two action were about winning the Presidency and helped bond the morning nation to Johnson with the effect of the landslide victory in 1964.

Bombing the dikes in North Vietnam was also considered. But it was abandoned as causing too many casualties.

At some point, you have to consider the international political fallout of winning a war at all cost. Killing 100k-1M civilians by rupturing dams would have been a PR disaster and would have seriously set back US efforts to contain Communism on the world stage.

Regardless of the amount of deaths, even if they are all enemy soldiers, using atomic weapons first will carry a massive stigma. You might somewhat get away with it if you are facing an existential threat to your country, say if Israel had done it in 1973, as some suspect they told the US they would do if losing. Or within Western Europe in the 70s or 80s in case of a massive Soviet invasion.

Done on a tactical level, the bomb wouldn't have had long term strategic (war-winning) impact. It was considered, I believe, at Khe Sanh, as the US didn't want Dien Ben Phu 2. But the political fallout would have been massive.

Done at a strategic level, ie. bombing of NV infrastructure, it might have worked, but the political cost would have been that much greater and escalation with the USSR could have been a possibility.

These insurgency wars are extremely hard to win in general and best avoided. As a big part of winning them is the elusive hearts and minds, excessive civilian casualties need to be avoided at all cost.

On a similar vein, B52 bombings in Cambodia severely damaged the US' image.

Not using nukes was the right call and precisely the reason why the ultimate war-waging authority needs to be held by politicians rather than left to the military. The US "lost and lived to fight another day" and Communism collapsed 20 years later. That might have turned out differently given a world that had massively rejected US values and alliances.

The ostensible war aim was to keep the South Vietnamese people free of communism so that they could thrive. Dropping nuclear bombs on their fellow countrymen in the North would have been self-defeating, even assuming that it worked at a military level and did not result in retaliation of any sort: (1) it's so close it would likely have contaminated South Vietnam with radioactive fallout; (2) it would have shown an immense level of contempt for the people the US was trying to help. A Vietnamese life wasn't worth less just because the person had the misfortune to be born in the North; it would have been unthinkable to compound that misfortune by dropping nuclear weapons on them. And I'm sure South Vietnamese hoped for reunification some day, just as West Germans did. You don't help people by slaughtering their brothers and cousins....

At that time, the US was concerned that a repeat of the Korean war could happen, with China sending in masses of troops... North Vietnam shares a border with China, just like North Korea. China wasn't overly fond of N Korea or Vietnam (in both cases, the initial supporter was the Soviet Union, not China), but when UN forces got near the Chinese border in N Kora, China sent in masses of troops... hundreds of thousands of troops.

So, defeating N Vietnam simply wasn't possible without risking Chinese intervention and massive loss of life on both sides. The whole Vietnam situation was a complete mess... in hindsight, it's hard to believe just how ill conceived it was, and how long it dragged on.

Also, the mid to late 1960's is when China and the Soviet Union had a major falling out, so it was difficult if not impossible to ascertain exactly what China would have done if N Vietnam were seriously threatened.

The use of nuclear arms so close to China's borders would have been extremely provocative: imagine China setting off nukes in Mexico. Unlike Korea in the 1950's, China had nuclear weapons in the late 1960's. Would they have used those weapons in response to the US using them so near their borders?

Best not to find out... an error of judgment could see Saigon wiped out, and all US personnel there.

Related note: the exit of the US from Vietnam closely parallels the unprecedented opening of diplomatic contacts with China, which had been essentially closed to the rest of the world at that time.

But, that's another very interesting story for another time.

By the time of the Vietnam war, the US nuclear weapons policies had changed radically from the mid-1950s (when nuclear weapons were seen as the logical option for ALL conflicts), and were considered viable only as a last resort and as retaliatory weapons in case an enemy would use them (or other weapons of mass destruction) first against the USA or a NATO partner.

So nuclear weapons were not a militarily logical or politically acceptable option, even though no doubt some in the chain of command did draw up plans for their use had their use been authorised by the political leadership.

But given the politicians' demand for ever more surgical, low impact, strikes against ever smaller authorised target areas, that was never going to happen (at some point the white house was listing not just what enemy bases to target and avoid, but which individual gun emplacements could be hit, leaving US strike jets to have to fly through dense areas of SAM sites and AAA batteries to reach their targets while not being allowed to defend themselves or preemptively take out those sites because the WH was afraid there might be Soviet or Chinese troops manning them).

Without the political resolve in Washington to win the war in Vietnam the US military, which could have easily pulled that off without resorting to nuclear weapons and with the forces in theater during most of the Vietnam war, the war could never be won with any weapons. This seriously demoralised many in the US military btw.