That misses the bigger picture, though. If the US hadn't supported Saigon, then how credible do you figure our support for the
30 September Movement in Indonesia would have been? Without in intervening in Vietnam, there would have been an unbroken chain of pro-Communist nations from the Asian coast to New Guinea. Astride the oil supply routes to Japan from the Middle East. I don't think I need to explain what kind of leverage that would have given China. Plus, there would have been a lot of temptation for China to make trouble in Northern Thailand, where there was a lot of simmering anti-Bangkok feelings (as evidenced in the
1973 uprising.) By making a stand in Vietnam, it forced Mao to turn the Cultural Revolution inward... but it didn't have to turn out that way - all of those red book waving mobs could have been sent South if we had given them the opportunity to do so. All in all, I think we would have been drawn into a conflict somewhere in Southeast Asia at some point in the 60's.