Question #1: Whose responsibility is it to declare war according to the US Constitution? Look it up.
Question #2: And why didn't they? Let me help you with the answer: Because they, and their Democrat majorities since 1954 would have been expected to authorize all that it would take to win it... and they, the Democrat politicians, had no intention of winning it, that's why. Given what he had to work with and riding the electoral blow out of 1972 the best Nixon could come away with was "Peace with Honor" but only after he took that electoral mandate forward 1 month later and bombed the crap out of Hanoi in Dec., 1972. POWs came home in Jan 1973, rememeber?
When Nixon was bloodied by Watergate, and after we had largely gone home, who was it that withdrew funding for South Vietnam, that saw the entire nation fall to the Commies in 1975, and destroyed our military intellegence structure that dogs us more than 25 years later? After, you answer that, who is it then that should be held equally as complicit with the failed outcome of the war in SE Asia and the "Killing Fields" as the Commie perpetrators themselves?
Question #3: So why do you ignore "them"?