By the late 1960's, Russia had it's own nuclear arsenal to rival the U.S.'s This was the basic reason why the cold war was, well, cold: The concept of mutually assured destruction. If one side starts slinging nukes around, the other side will respond, and the entire world ends up in a new ice age.

​

Why does this matter with regards to Vietnam? The Vietnam War was, at a basic level, another regional war between superpowers. The U.S. backed the (supposedly) democratic South Vietnam, while North Vietnam was a communist state backed by China and Russia. The general belief among U.S. leadership was, for most of the cold war, that nuclear weapons should only be used as a last resort, and that they were doing their job by simply existing, sitting in silos and aboard submarines: They were a deterrent. Very few people actually wanted to use them.

​

It was also understood that using a nuclear weapon against a communist backed state would very likely have resulted in a dramatic escalation of the Vietnam war, even if North Vietnam itself backed out. That the U.S. would be willing to use a nuclear weapon would, by most analysis of the time, have resulted in China and possibly Russia overtly sending their own troops into North Vietnam against the U.S., which was considered by most communist states to be Imperialist Aggressors. Essentially, nukes weren't used in Vietnam because there was the very real possibility that doing so would have started world war 3 and doomed both the U.S. and Russia and most of the world to death over a small regional conflict.

​

Also, Vietnam wasn't really that important. That the U.S. and other allies invested so heavily into protecting the 'democratic' South Vietnam was really little more than a show of force, intended to show the Communist countries that the U.S. was willing to support democracy anywhere in the world. It wasn't initially anticipated that North Vietnam would prove so effective at fighting back, which led to a build up of troops and equipment until finally the U.S. realized that they would either have to have another massive troop surge, or withdraw. They chose the latter. The former would have been almost impossible considering the massive lack of support for the war at home. As it was, there were massive protests and civil unrest. Another build up of troops would have been a domestic nightmare.

​

Another issue to consider is that use of nuclear weapons would have made the U.S. look terrible to the world as a whole, even to its allies. This was an issue during the Korean War as well, when a plan was formed to essentially create a nuclear "no mans land" just over the North Korean border in China to prevent reinforcements from China, which were at the time streaming over the border. It was realized that this was a terrible idea, as the U.S. showing that it was willing to kill potentially millions of people and use nuclear weapons over another low intensity regional conflict would have very likely made them enemies of many non-aligned countries and quite possibly deprived the U.S. of allies such as the U.K., France, etc.

​

TL;DR: It wasn't worth risking a start to world war three over a strategically unimportant country in south-east Asia.