Mark Moyar argues in his controversial monograph Triumph Forsaken (Cambridge University Press: 2009) that the Vietnam War did prove beneficial in preventing Communism from spreading to Thailand, which was one of the stated aims of the war. Moyar, of course, believes that the United States could have achieved even greater aims--namely the stunting of Communism in Southeast Asia altogether--had Lyndon Johnson and other policymakers properly executed the war. The second half of the work, which covers in detail the period after 1965, Moyar has yet to publish, so maybe he will yet provide a convincing argument for this conclusion. As of now, the academic community has not accepted Moyar's revisionist account.

The strongest evidence against this line of reasoning, I think, is the way relations between Vietnam and its Communist neighbors disintegrated once the United States withdrew from the region. The threat of monolithic Communism that had compelled Eisenhower to intervene in Southeast Asia after the defeat of the French in the 1950s proved a fiction that American power had protracted by meddling in Vietnamese affairs in the name of anti-Communism. For two decades, between the Geneva Convention of 1954 and the eventual American withdrawal under Nixon in 1973, the United States combated the spread of Communism from North Vietnam to South Vietnam, only to see Vietnam go to war with two of its Communist neighbors before the end of the 1970s. Nationalist tensions arose quickly after Vietnam secured its new borders, leading to a war with Cambodia largely driven by fears that Vietnam would try to reunite Indochina into a single nation dominated by Hanoi. China, which had provided Hanoi with arms to fight the United States during the war, invaded Vietnam in 1979 in an attempt to check Vietnamese aggression against Cambodia. The fact that all three powers were Communist did not make them a single power that would continue to spread like falling dominoes across the globe, it turned out. It's very difficult for diplomatic scholars, therefore, to claim that the war ever had the strategic importance that Johnson and his predecessors imbued in the conflict with their anti-Communist reasoning and rhetoric.

If anything, American intervention in Vietnam proved far more beneficial to the Soviet Union than not intervening would have. Counterfactuals are, of course, very tricky. But the USSR was the only party involved in the war to accomplish all of its objectives. The US failed in its attempt to protect and build a non-communist Republic of Vietnam. China went to war with the communist Vietnamese regime in 1979 and suffered a humiliating defeat, therefore frustrating all of China's previous attempts to bring the Hanoi government into its orbit. The Saigon government ceased to exist. And although the DRV (or North Vietnam, if you prefer) won the war, it was a Pyrrhic victory. Ho Chi Minh and other communist leaders had long dreamed of a unified, independent, strong Vietnamese state. But the only thing that worked in postwar Vietnam was the military. Basic infrastructure in the north was either mostly destroyed or 30 years out of date. There were multiple famines in the 1980s. Unexploded ordinance was (and remains to this day) a huge problem. The war claimed millions of Vietnamese lives, both military and civilian. And the government enacted hardline policies that only made many problems worse. Not until the 1990s did Vietnam start liberalizing its economy much like China did under Deng Xiaoping. But while that decision led to a better standard of living for many people, it ironically meant that Vietnam became much more of a capitalist society than the wartime communist leaders had intended. Moscow only had two goals: make the war unwinnable for the US and bring another nation into the Soviet orbit. They accomplished both goals handily.