[chasil]

I am not deeply read on this subject, but the loss of President Diem profoundly impacted South Vietnam in an adverse manner for the U.S. efforts to maintain the country.

https://en.wikipedia.org/wiki/Ngo_Dinh_Diem

There was also great turmoil in the U.S. at the time (as we remember with the approaching DNC convention in Chicago). Nixon implemented an effort to distance the U.S. from the situation.

https://en.wikipedia.org/wiki/Vietnamization

Watergate closed the door on efforts to maintain South Vietnam.