The Domino Theory suggested that one nation becoming Communist makes it more likely that others will too. Without American involvement, the South would quickly have fallen to the North and the nation, united under Communism, would have joined the Communist bloc. The nation would then add its strength to the forces opposed to the West and to democracy, and become a threat to its neighbors – in this case, the unstable regimes across Southeast Asia. Communism would spread through these countries, to Indonesia, from where it would threaten Australia. The USA had to oppose this, because the North Vietnamese were being helped by China and the USSR. In the long term, this expansion would threaten the USA itself, by which time it might be too late to act: this was self-defense. The containment policy gave the US the ability to oppose the Communists in battles that took place away from areas important for the USA’s national interest (like US soil). It is true that Communist Vietnam posed no significant threat to the region post-unification: but would this have been true if the North had been engaged in years of bloody conflict? The war gave Thailand, Singapore, Malaysia and the Philippines a breathing space so that they could not be overrun, as Cambodia was. At least in part, they owe their existence as vibrant, free, capitalist nations to the USA’s stand in Vietnam.
The interference in another nation’s sovereign affairs was a violation of international law. It is only legitimate to attack another nation if they attack you, or possibly if you genuinely think they are about to: nobody would ever suggest that the North Vietnamese threatened the American people in any way whatsoever. Once involved, the USA bombed Cambodia and Laos, too – breaking more laws. These violations of the law undermined the western bloc’s claim to moral superiority. Furthermore, history shows that the domino theory was wrong. This was a civil war, one that would have been over far more quickly and with much less bloodshed had the USA not interfered.
Many of the opponents to the South’s government were not Communist, but rather nationalist or anti-Diem. But all opponents served to undermine anti-Communist forces and helped the Communists. It is true that Diem was far from perfect – but in the fight against Communism, necessity forced the West to deal with many people whom they did not fully agree with. Vietnam was no different.
It is false to present the USA’s actions as the restraint of Communism. Many non-communists in Vietnam opposed the Southern government, because it was so corrupt and so oppressive. Diem wasn’t elected, he was appointed by the Emperor – he cancelled elections, made himself President, and refused to initiate post-colonial land reforms. Many people yearned for national independence, and they saw Ho Chi Minh’s Viet Minh movement as being able to deliver it. The West viewed all opponents of the Southern government as one group, radicalizing even those who weren’t Communist but wanted independence. Even Ho Chi Minh was more of a nationalist than a Communist, but he was forced to rely on Communist allies because the leader of the capitalist world turned against him. If the Allies had honored their promises for independence, maybe Vietnam would have become an ally in the Cold War.
Source: Debate: Vietnam War