One thing that the Vietnam War showed is that there are no set parameters that can foretell the outcome of a war, such as the article in The Computer That Predicted the U.S. Would Win the Vietnam War A cautionary tale about the dangers of big data explain with interesting facts on big data history.
However, one piece of information that I could not directly find in my research is if there was any official or media interview declaration where one or some personalities made an affirmation on the assuredness of US victory in the Vietnam War.
Is there any citation, picture, transcription or online archive that contains any such citations, if they exist?