Argument #1: The Domino Theory
The Domino Theory suggested that one nation becoming Communist makes it more likely that others will too. Without American involvement, South Vietnam 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—the unstable regimes across Southeast Asia. Communism would spread, threatening Australia. The USA had to oppose this, because the North Vietnamese were being helped by China and the USSR. This was self-defense. The containment policy gave the US the ability to oppose the Communists in battles that took place away from 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 time for self-defense, so that they would not be overrun, like Cambodia. They partly owe their existence as 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 if you genuinely think they are about to. 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. History later showed 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.
Argument #2: Opposition to the South’s Government
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. Diem was an imperfect leader, but in the fight against Communism, necessity forced the West to cooperate with many people with whom they disagreed. 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 corrupt and 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 wanted national independence, and they thought Ho Chi Minh’s Viet Minh movement could deliver. The West viewed all opponents of the Southern government as one group, radicalizing even the non-Communists who wanted independence. 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, Vietnam might have become a Cold War ally.
Source: Debate: Vietnam War