Answer:
the reason was to purportedly stop the "domino effect," of communist nations overtaking other countries or the countries near them becoming communist also. the united states being more supportive of a non communist and non socialist ideal. but aside from strict "communist," or "socialist," ideals being opposed, the fear was also present that north vietnam, regardless of political labels, was not going to be open to democratic expression. when the "gulf of tonkin," incident occurred in 1965 the USA became much more involved and it from them on out until 1973, was a actual war. president kennedy approached the vietnam situation as one which the USA should be involved but doing this meant much military advisory role and not as much actual ground combat as would happen in the late 1960's. military special forces were sent to south vietnam in the early 1960's, during Kennedy's administration in a training mode. their purpose was to train and advise the south vietnamese forces. president johnson and president nixon had a different role in that the war was a actual war full fledged beyond doubt. so the administrations, both , committed to American forces ,troops on the ground in the 1965-1973 time range.