I've been watching Ken Burns' Vietnam documentary, and reading Tim Weiner's "Legacy of Ashes". Both put emphasis on American leaders treating the "Domino effect" of the surrounding nations following suit should Vietnam succumb to the communism as a foregone conclusion and cite it as the main reason for going to war.
However, neither Weiner nor Burns question this logic. In fact I've never in my life seen anyone question this logic. But with the benefit of hindsight, how likely would it have been that all of Southeast Asia would turn communist just because the relatively small Vietnam did? In theory, shouldn't the massive China turning red have been enough to set off the domino effect in Southeast Asia?
I live in Southeast asia. My parents came of age during the time the domino theory was at its peak.
Honestly speaking, it did seemed very real. Remember, every country in SEA had some kind of communist insurgency at some point or another. At the height of the Vietnam war, you need to remember, most nations were barely a decade old.
Based on my parent's memories, communist vietnam seemed to be the strongest nation and the most likely to steamroll over Thailand and malaysia.
In fact, there is some ground that the fact that the US got involved in Vietnam actually bought time for the other nations to achieve 2 things:
develop an actual economy with jobs and reasonable wages such that it convinced the local people that communism was not the only path
Build up their own armies.