You make a very good point, one that I should have added to my previous post.
I guess the Vietnam War was the turning point.
“We are fighting to stop the spread of communism.”
“There is light at the end of the tunnel.”
Etc.
That war turned most Americans into doubters and cynics.
Once we got out of Indochina, it paved the way for The Killing Fields.
Ironically, though it was the Vietnamese Communists that put a stop to it.