The Domino Theory was that if South Vietnam fell,  others (Laos, Cambodia, Thailand) would follow.  When South Vietnam fell in 1975,  Laos and Cambodia fell in sequence.  Thailand, fortunately, didn't follow.  The US goal in going into the South in 1965 was to prevent the dominoes from falling,  S. Vietnam included.  The NVA and VC couldn't defeat us on the battlefield, but were successful in Congress.