The same reason it saw fit to invade Afghanistan and Iraq during George Bush jr's admin............


EM: Do you feel in any way responsible for the War? Do you feel guilty?
McNamara: I don't want to go any further with this discussion. It just opens up more controversy. I don't want to add anything to Vietnam. It is so complex that anything I say will require additions and qualifications.
EM: Is it the feeling that you're damned if you do, and if you don't, no matter what?
McNamara: Yeah, that's right. And I'd rather be damned if I don't.