Then the cold War ended and they realized they didn’t have to masquerade their missile defense programs as human spaceflight anymore, and cut the budgets for all that stuff.

You should clarify the title to say “In 1969 VP Agnew planned for a manned landing on Mars between 1983 to 2000.”

There was a whole plan for space colonization.

https://en.m.wikipedia.org/wiki/The_High_Frontier:_Human_Colonies_in_Space

It will always anger me how close we were to a For All Mankind timeline.

Direct link: https://en.wikipedia.org/wiki/Space_Transportation_System