They had different views about the role of American troops. At the time of his death, Kennedy wanted our troops in support and training roles, not direct combat. Johnson started out the same way, building up American troops in support roles. But, the more American troops in Vietnam, the greater the target and provocation they were for the Commies. By 1965, Johnson had to committed them to active combat in order to protect themselves, because the South Vietnamese Army wasn't up to the job.

While that was the difference between Kennedy and Johnson, they had one thing in common: both thought Vietnam was worth a major commitment of American power. Had Kennedy lived, and had he built up American troops in the same numbers as Johnson, he would have faced the same dilemma of his own making.