Harry,
Root causes for the war began in the 1950's under John Foster Dulles and Richard Nixon and of course the military-industrial complex.  Yes, the M-I complex controlled Democrats as well as Republicans.  Texas (think General Dynamics and the many huge military installations) was home to LBJ and Sam Rayburn.  They also were puppets of the M-I complex but LBJ in 1968 after Tet knew the war was unwinnable and quit.
Yes, Nixon was an ensign in the Navy but saw no combat.  He wasn't like Jack Kennedy who got shot out of a PT-109 in the Pacific.
If Kennedy lived, he may well have taken all the "advisers" out of Vietnam as he did with the ill-fated Bay of PIgs adventure in Communist Cuba.  That adventure was instigated by the CIA in the Eisenhower-Nixon administration.
Yes, Nixon, an admitted crook, was responsible for the war.  That's why he formed the White House Plumbers to burglarize people like Daniel Ellsberg (Pentagon Papers) and George McGovern (DNC at the Watergate building).