Domino theory is such BS. Most communist movements in the cold war period were literally just organic movements in the country; people wanting something better for themselves. What do you know, when a foreign colonial power owns most of the means of production in your country while your people starve, communism looks pretty good. While some may have accepted material help from the USSR (usually after the US/CIA had intervened), they definitely didn't want Soviet influence over their heads.

Edit: also, according to Matt's shitty theory literally anything is justified because "communism is worse". Kill a few million folks in Vietnam? Well, communism would have killed MORE and no I will not show my work. Demonizing socialist countries and making up body counts serves the capitalist empire because it rationalizes the bad shit they do.