I understand the fear of the spread of socialism during post-WWII world history, but why did the US, and other nations, put so much effort, resources, and lives into relatively small countries that showed widespread support for socialism? Did experts and advisers truly believe that the spread to smaller nations would have a negative impact on the global economy, or peace on a meaningful scale? What precedence did they have for this? Did any of these ideologies change in the aftermath of the Vietnam war?
As a follow up, were the intentions at the government level due to the same fears instilled into the general population regarding communist movements, or were there other motives at play? What was the reaction of other USA aligned countries (government level, not general population) to the choice to abandon the 1956 election? What about their reaction to the aftermath of the Vietnam war? And finally (I'm sorry, I'm broadening, but I obviously don't need every question answered by one single poster) what about the reaction of countries involved in the non-aligned movement (Yugoslavia especially)?
The company line was that communism was a single, Moscow (sometimes Peking-)-directed entity that would wrest the world from capitalism one nation at a time. Socialism held a particularly strong appeal in former European colonies as the doctine explained colonisation as natural expression of capitalism. Also, people were poor and often exploited by landlords and rulers, whether colonists or locals. This meant the world's 'trouble spots' tended to be former colonies.
In 1945 the US was ambivalent at best about European nations returning to their colonies. Countries like France and Belgium had had their economies destroyed and they were not in a good position to re-establish their colonies and expend sums on security and development, but at the same time they wanted to restore their pride and they often had emigre populations and companies pressuring them to defend European rule. France re-conquered Vietnam in a bout of nationalistic hubris in 1945-6, then spent the next five years trying to hold back Ho Chi Minh's nationalist/socialist movement with a shoestring army.
In 1949-50 everything changed. The fall of China to communism (after the US had expended vast amounts over many years to prop up Chiang Kai Shek), the Soviet-backed invasion by North Korea of its southern neighbour and the UN/US response, the Berlin airlift, and the Soviet detonation of nuclear weapons much earlier than expected all led to a certain sense of panic. There was a late realisation of the darkness of Soviet totalitariaism, and a desire in the West to band together as they had against Hitler. This was also the era of McCarthyism and anti-communist populism: it was not a time for a career-minded person to espouse the view that Ho Chi Minh was just a nationalist, Vietnam just a backward little country.
The French were thus able to re-cast their colonial war as a war to defend Western values against godless communism, directed from Moscow. But the war was already unpopular in France itself, though nobody wanted to be the one to pull out.
By 1954 the French had stepped up the war in Vietnam and were receiving considerable US help in money and equipment. The final French gambit to win the war at Dien Bien Phu, a battle which took place against the backdrop of a four-power (US, France, USSR, China) conference to end the war, was a disaster as the garrison was surrounded and, after an intense battle, defeated. This gave US authorities some time to consider their response to both saving the French (use of nuclear weapons was seriously considered), then to craft a response to French defeat and withdrawal. This led to the grooming of Ngo Dinh Diem as a palatable alternative ruler, but his credibility next to Ho Chi Minh, who had humbled the hated French and liberated the nation, was always going to be negligible.
Through the 50s, as anti-Western and post colonial nations embraced Soviet support and socialism to varying degrees, the notion of the 'Domino Theory' developed in Asia: once one country falls, as Lyndon Johnson famously asked an audience, 'ask yourself what's going to happen to all the other little countries'. Many of them already had strong socialist movements.
Most European nations were grateful to be out of their colonies in the 1950s. Often, the home governments were very left-wing and saw the nuclear threat and weapons spending- after millions had suffered in the war- as a waste and a terror that had to be dealt with. They were happy to be out of Asia, and did not want the US to get too distracted there as the real issue for them was NATO and building a counterweight to the Soviet Union. Australia and New Zealand were very happy for the US to be fighting in Vietnam, however.
A really hood source on this is Embers of War by Frederik Longevall.